Played through the demo over the weekend, and I have to say that after all the hype, Hellgate London did not deliver.
First the graphics...the game world feels very repetitive and static, and the textures and graphics are 2 years behind the power curve...the random dungeon approach worked well for Diablo given the maturity of graphics at the time, but this approach to level design does not work in a fully realized 3D world.
The enemy AI poses absolutely no challenge. I don't recall the character class, but I chose the military shooter class in the demo, and chose to play from a FPS perspective...for this class, you can stand off at a distance, and quite easily pick off the demon hordes...by the time the AI reacts to your attacks, and start to charge you, they are dead.
I switched to a traditional 3rd person perspective, thinking it might change the feel of the game, but it simply made things more frustrating...not harder or more immersive.
The gameplay mechanics are RPG-lite at best, which again worked well for Diablo, but is not translating very well to PC gaming of today...given the advances in gameplay mechanics since Diablo's release, players expect a more immersive gameplay experience, even for an action RPG...running around fulfilling FEDEX or kill X of Y quests doesn't excite me anymore, and gaining the best loot isn't enough of a motivation for me to continue playing a game.
The music and sound effects didn't really add much to the gameplay experience, but I think BioShock spoiled me in that regard.
Overall, I have to say that Hellgate London was a bit letdown...which is unfortunate, because I did enjoy the Diablo series, and had high hopes for this game.