Say it with me: "It's not another Diablo clone. It's not another Diablo clone." Okay, on the outside maybe it is a little, and I see why people think that since a few of the same developers who whipped up this hellacious game set in a futuristic London backdrop did a little something-something called Diablo 2. Yea, it's easy drawing comparisons between the two because both games have different settings, different classes with different skill trees -- oh freakin' hell, that's Diablo 2 slapped right-on-top of Hellgate: London!
I laugh giddily at those who think it's the same game without even trying it first. You should decide if the game is fun as hell to play and the best way to do that is opening the gates and checking out the Hellgate: London demo. If a download is out of the question, but you are still interested in the different classes; a developer-diary posted on Pro-G keeps the gates to hell open by discussing Hellgate: London's character classes, the factions, weapons, and the hellish cycle classes went through to get to where they are today.