I finally completed Hellgate: London tonight with my Engineer character. I was actually pretty close; I just had to pass five trials, which were the most interesting part of the process as they were set up as a good-vs-evil battle with different rules in each one, usually revolving around capturing and holding control points.
After that it was off to another station, where there were some more side quests that could be run, and then off to the titular hellgate for the final battle. There wasn’t anything particularly special about it though, aside from being a bit more hectic than usual due to dense monster spawns, and the end is clearly a setup for a sequel or expansion pack.
I then played for a bit on Nightmare mode, making it to Covent Garden station again, but the difficulty didn’t really increase that much, and it felt pretty much like a continuation of where the normal mode left off. It supposedly gets much harder in the end though, with the final boss being extremely difficult and/or time-consuming (only recently has someone claimed to have finally killed it solo).
Though I can technically call HG:L done now, it’s very much like Diablo 2 in that it invites a lot of replay through different character builds, different difficulties, loot hunting, etc. But for now I think I’m going to shelve it again until more patches and added content comes out.