b) Someone who actually knows what he's doing, but wants more of challenge.
And then there's c): Someone who reads the Wiki before entering into any new area or taking on a quest.
I think that your c) is covered by my b). If you read wiki, you know what you need to do to achieve something.
Fair enough, but it's kind of vague what you mean by "someone who actually knows what he's doing" in my opinion; I have played MANY RPGs, starting with the Bard's Tale on C64 back in the day, Dragon Wars, Planescape Torment, all the Baldur's Gate games and mods, the whole Fallout Series, Icewind Dale, almost all of the Elder Scrolls Series, Realms of Arkania series, etc., so I think I 'Know what I'm doing" but then there's a different kind of "knowing what I'm doing". If I've gone through the whole game with a companion who can heal me (Hirge) and then I'm faced with enemies with both shock and fire damage potential with NO warning at all about what I'm facing, AND no companion, well, I don't want anyone to hold my hand, per se, but I WOULD like maybe Tremadan's hologram to at least say something that might clue me in to what I might have to prepare for. Same with the Sewers. I realize that by the time you're ready to tackle the sewers you probably should have decent equipment for resistances, but I keep most of the specialized ones in my vaults, until I know I need them. That's why I had to reload a save for my warrior from 2 levels below current to re-spec for the Sewers, because the Elemental Plane of Fire was killing me SO quickly and I had NO idea going in what I needed to have to prepare for it.
A simple dialogue with an NPC would be REALLY nice:
"Beware, if you enter the sewers! Toxic fumes and ice are dangerous there. And fire! Never before have you felt such heat!"
See? A simple dialogue like that would have made the Sewers MUCH more enjoyable, IMO.