Plants Vs Zombies was my most recent guilty pleasure, but let's keep that to ourselves.

Oh man...list time of MY secret gaming loves:

ALL Lucas Arts Adventure Games (I have most of them)
Star Wars Dark Forces/Jedi Knight series (ALL of them >.>)
Final Fantasy IV-VI
Doom 1-3 (yes even 3 I admit it)
Chrono Trigger
Earthbound
Legend of Zelda: A Link to the Past
Radio Active (music trivia game)
Dust: Tale of the Wired West
Splatterhouse 1-4 (yes even 4, something about hearing Jim Cummings saying "show them why we call this the SPLATTERHOUSE MOTHER FUCKER!" gives me the giggles)

As a young lad, I got Rayman for x-mas on PSX, sort of the last minute gift from my grandmother. I was surprised to find that it was insanely colorful, and my brother, father, and a few friends. who were some of the biggest homophobes I had ever known years before they realized how useless it was to be one, would always comment on the colorful nature of the game and all I would hear is "Gay this, gay that." So I would play the hell out of it behind closed doors, so as not to hear the bullstuffing escaping from their mouths.

The necromancer on the cover of the Guild Wars GOTY edition case. Damn!

I'm very curious, what exactly is guilty about it?

Hm, hard to tell... considering that OP seems to mean the universes rather than the games themselves... I certainly adore the Witcher and Stalker universes. Kane & Lynch on the other hand are probably my favourite protagonists that I absolutely want to see in a third game (Lara Croft and the Guardian of Light does not count). And I've been a huge fan of anything related to Warhammer since my early teens - the love shrunk a bit over the last few years but I still think that Warhammer is THE universal fantasy franchise and Dark Omen and Dawn of War happen to be among my favourite RTS games of all time. I'd also love to add the Hitman franchise but since even the devs don't seem to care much about consistency it's hard to like the "universe" - I'm always happy to see agent 47 though (as long as he doesn't see me).

But honestly, as for the Warcraft universe - I hate it. I think it's stupid, generic and uninspired.
Maybe its cuteness.

I still log onto various text-based MUDS. Whether it's the occasional Battletech game, Nanvaent, Medievia, or any host of other games, it's hard to find games that draw me in quite like a game where my own mind is the graphics engine.

Those words scrolling up the screen gets me excited every time!

I'm actually not very fond of adventure games, but I've played an inordinate amount of them regardless. For some reason I always find a reason to play a particular one.

Shh, don't tell anyone.