It's quite possible to be a casual roguelike player. I get into them for bursts now and then and then I get distracted. I've never once won any of them or come particularly close to it. I also can't get into NetHack at all, which saddens me because of how much people seem to like it, but I'm so used to Angband and variants now that it really does not make any sense to me. Not even the controls.
I second the recommendation of DoomRL, which is far better than a roguelike based on an FPS has any right to be.
My personal preferred Angband variant is Steamband. TOME is deeper but in my opinion overcomplicates things (I'm not a great fan of wilderness or multiple towns in any roguelike to date for one thing, although there's got to be a good way to handle such things somewhere.) Steamband offers a great theme (steampunk splashed with tons of references to Victorian-era literature, particularly of a science-fictional nature), an excellent skill system, and a lot of cool little things like steamware upgrades and racial spells. And while development has been lagging, it *is* still active.
Also - still somewhat early in development, but has a lot of potential:
Incursion: Halls of the Goblin King. D20 OGL based with insane amounts of character development and explicitly rules-based interactions (as opposed to NetHack's special-case interactions). A lot of planned content isn't in yet, it's a bit buggy, and it's really, really hard to survive with many character builds. But already it's pretty amazing.
If you're not averse to paying a bit of money, ChunSoft's Mysterious Dungeon series is worth noting, if not often as deep as the best freeware games. Translated ROMs of SNES titles Taloon's Mysterious Dungeon and Shiren the Wanderer (I'm not sure about the former name, but it's something like that) are available. English-language releases include the PSX Chocobo's Dungeon 2 and Torneko: The Last Hope, PS2 release The Nightmare of Druaga, and GBA/DS release Pokemon Rescue Team. Possibly others. And while I don't believe ChunSoft had anything to do with it, Izuna the Unemployed Ninja for DS is also a pretty engaging roguelike.
Also on the commercial, but in this case no longer sold front: JauntTrooper - originally a Macintosh shareware series (Mission Thunderbolt and Mission Firestorm being the two entries, so far as I know), but the first of the two was ported to Windows. Both can be downloaded from Home of the Underdogs, although annoyingly there doesn't seem to be any source of the Macintosh version of the first mission, something which seems like it would be useful if one were to want to import one's character into the subsequent game (as is possible). Maybe the second game supports Windows save files, though. Dunno.