Storyline
I only got to Act 4 - "...All the live long day" quest, but I'll say that the storyline is pretty much useless and non-existent. I'm a fan of Blizzard North's decision when making Diablo 1 (and D2) to not focus too much on lore and character creation character building (aka DND style - Let me spend 2-5 hours making a character) and just let people "Pick a char, Name a char, and let's swing at the skeleton", but at least for D2, the basic storyline components and Act cinematics were small but impactful. For HGLs, it just felt like they were making content up just to fill things up.
Quest Lines
This is connected to the Storyline portion, but I don't feel the quests are that good. There's a lot of side quests and there seems to be daily quests like in MMOs, you can also only have a certain amount of quests even registered at any given time (Like 5-7 around there), but the quests usually consist of grindy type of quests: "Get 5 keys", "Go kill this guy in this place". I'm not against having these types of quests, but it just seemed like a majority of them were that type. Even main storyline quests consisted of some of these types of quests as well.
There was one quest that I hated and didn't understand it until I looked it up, which was a quest where you have to guide your troops to safety or something like that. However, the game would change your camera to almost a RTS style approach and you are basically clicking a location, and your troops would automatically try to get there (Through their own AI). It was a very frustrating and disorienting experience and should just be removed from the game completely. I pretty much ended up following other people's advice to just move your camera directly to the "Beelzebub" monster and just nuke him. When your people die, respawn, and re-do the whole strategy. Your troops don't matter. Just keep nuking Beelzebub until he goes on fire and then the fire damage will take him out relatively quickly.
Overall, having quests that you can come back to is not bad, but I feel there should be a smaller amount of quests in a game where each quest is impactful, fun, and meaningful. Not just random filler content that makes me feel like I'm just wasting my time.
Randomization
Levels
The game has random levels but since the game doesn't have many tile sets, and it all takes place within London and its Underground, the game unfortunately feels and looks the same pretty much everywhere. There are a few places where things look a little different (like if you are in an Ancient Relic or a Hellrift), but for the most part, it pretty much looks the same, which gets old really fast. I tried to justify this by saying "Well, it's within the city of London, it's not like you are traveling to the outskirts of the UK, or some other country", but yea even with me trying to justify it, they could have had a lot more different tile sets even if it was just the base game. Like when Diablo 2 came out (pre lod), each act was a completely different tileset. Even if things looked a little familiar (Den of Evil, Caves, Underground Passage, The Pit) or (Blood Moor, Cold Plains, Stony Field, etc etc), the game is small enough, and had enough of a context shift frequent enough in tilesets even within the same Act, that it didn't feel as boring. Once you switched acts, at least the next Act was different enough that it allowed you to reset - mentally. Through the WP system, you could just "go to another act" aka "switch the tile set completely" if you were tired of playing within a particular tileset. However, for HGL, everything is in the city and the underground, so pretty much everywhere you go unfortunately looks the same.
The hellgate rifts was something I really liked. At random locations in the world, Hellrifts would spawn (A red portal), and you can choose to go in there, kill some monsters, and there will usually be some chests that can drop loot. Sometimes in the middle of the central "palace" in the hellrift, there could be another chest, or there could be a monster, or a portal to another level, sometimes it's called "Ancient Relic"
or something like that, this takes you to another location in Hell where it seems like it's some very old gladiator "palace of the demon gods" type of thing. This was pretty cool. In there it is randomized as well and you can find some more chests somewhere in the map.
However, all the hellrifts have the same exact layout. Randomization within the hellrift would have been nice, and of course making each of the Hellrifts, and any connections more meaningful.
The game also has "Passageways" as well that can randomly be located in many of the game dungeons. There can be different passageways as well. Once you enter a passageway, it will reveal what type it is, such as a "Treasure Passageway", "Elite Passageway", etc. Sometimes a passageway can contain another passageway that is randomly generated in there. This was pretty cool as well. But overall most of these locations weren't as random as I wanted them to be and also didn't give enough positive item feedback for me to feel like they were worth it. Sure with infinite amount of time I would do it, but given that I was getting bored with the game, I decided to start skipping a lot of these side stuff in order to try and even finish the game - at least once.
Itemization
Weapons are interesting in that you also have randomized weapons and properties, and you also have weapons generating with random modification slots, essentially an improved version of D2's socket system, where you don't only have sockets, but also a socket type (Batteries, Relics, Etc). In It Lurks Below, Brevik used sockets and socket shapes. There are also tiered equipment as well, and it was interesting to see that there Legendary Items (Which are 3 stars and brown), but there is actually a tier 4 item which is Unique and seems to be gold color. So you have something like Common (White/Grey) -> Enhanced (Green, 1 Star) -> Rare (Blue, 2 Stars)-> Legendary (Brown, 3 Stars) -> Unique (Gold, 4 Stars). There are also things in town to allow you to unsocket items (While keeping both the weapon and all of the modifier parts), there is just a small fee. I didn't feel like any of the properties were particularly meaningful to me too much, some of them of course brings a smile to my face (like Movement Speed increase, or + Luck) but a lot of stuff is either confusing or doesn't seem as helpful. For me, if a game doesn't have itemization that I like, that pretty much kills a lot of the entire game for me since these types of games are all about items.
One more thing is the crafting. I didn't feel any of the items that were available in the crafting vendor were really important, and I pretty much dissembled all of my spare equipment. Originally I placed all of my crafting materials in my stash so that I could save inventory space, however, the game doesn't have any sort of auto-combine functionality so I pretty much kept all of my crafting materials in my inventory so that when I dissembled, the game would "auto-combine" into what's already in my inventory.
There is no shared stash in the game btw. This is a good thing and a bad thing depending what type of player you are. If you enjoy untwinked play, then this is really good (Same as Diablo 1 and Diablo 2 design). If you like twinking then it won't be as enjoyable.
Characters
I only played the Engineer and in the beginning it seemed interesting. It had a synergy system which definitely came from D2's 1.10, and Peter Hu worked on 1.10 as its main architect and also on HGL after he finished 1.10 and immediately went to Flagship Studios. But as I leveled up, the skills weren't that helpful, and I didn't really feel like investing points into not only the same skill that I unlocked, but even other skills in the tree. I usually ended up placing points into skills because I needed a minimum amount of points in a dependency skill before I could go to the next one. Very demotivating. The stat system is interesting in that you basically use it as a way to equip items. The stat pool is shared which means that if you have 15 str, and you have a piece of equipment that uses 5 str, and another that wears 5 str, and another that has 6, you would only be able to wear 2 of those pieces since you won't have enough total str to equip all of them at the same time. There are some bonuses that each attribute gives you, but from my experience, and from what other people in the community are saying, you pretty much should save all of your stat points until you get a piece of equipment that you want to wear. The engineer does have some "Bot" skills, where one of these "Bots" is pretty much a D2 Merc. As you put skill points into the Bot's tree, you can enhance it so that it can wear a weapon, an armor piece, or even a melee weapon. The engineer can have two bots at once so for me I pretty much wanted a Merc bot (That assists me in killing stuff and I can gear them up a little), and then a secondary bot that has some other ability (Slowing enemies down, them crashing into them and doing a stun effect, or you can have a Haste bot, etc). But yea, it didn't feel as satisfying as D1 or D2's skill system, and you all already know D1/D2's skill system could have used a lot more work lol. I was thinking maybe it's just the Engineer class and I should try another, but given all of the other issues with the HGL design, I don't see myself replaying this game again, but it was very important context for me to understand the before and after of Diablo 1/2 and also to understand some of the stuff these same developers did with their games and what lessons we can all learn from their experience.
Sound
In the beginning I was playing and listening to all of the gameplay music and the weapons and all of that, however, after a good 10 levels of just listening to bullets, I got even more headaches from the sound of the ammo. I tried to eliminate weapon sounds completely and just have game music, it helped, but what didn't help was that I'm pretty much listening to in-game music that sounds like heavy metal. I get some people like to slay demons with insanity playing, but it doesn't help LOL. I need some relaxing, Matt Uelmen Tristram guitar sounds. I ended up muting all of the game music and continuing on with my life.
Final Thoughts
Since I haven't been playing 3D types of games for a long time, playing 3D games now for me tend to give me headache. Eventually I'll get used to it (Even after adjusting for optimal screen distance), but 3D games, especially if it's like a FPS/RPG (Borderlands, HGL) type of thing definitely increase the chance of me getting a headache which is obviously not fun. I feel my brain has been optimized for flat, 2d (or fake 3d isometric *cough* d2 *cough*) games where things are much slower pace and I don't feel I'm getting a headache lol.
Overall, those are some of my thoughts about HGL. I think that if the game had more time to be developed, their business model was massively changed, and some other adjustments, this could have been a really good game. There were a lot of things that came directly from their experience with D1 and D2, and it shows, and they also introduced a lot of experimental ideas, probably with a bunch of them coming from MMOs. But unfortunately this is the reality.