Being somebody that only got into roguelikes a matter of months ago, I have several turn offs, probably brought on by modern gaming conventions that I'm used to.
Controls (1)
Yeah, I'm used to very simple controls. An xbox controller, or WSAD-mouse, with perhaps a few other controls. This is only a minor gripe though. I mean, I bothered to learn Nethack's control scheme, to make sure I played the game properly. ...Still took me forever to work out what the #meta key was though...
ASCII (1)
...Yeah. Sorry, but I'm not a fan. However if it was possible, the gripe would be less than 1. I've been playing a lot of Cataclysm lately, which is ASCII only, and there are many other games I play using that style. But despite this, I prefer tiles. <3 DCSS's tileset.
Cheapness (3)
This is my way of wording the way that I hate difficulty. I barely play nethack now, as it often feels very cheap to me. It seems like the game sometimes says 'Yeah, you're screwed now because you didn't manage to find a way to float earlier. Screw you.' Also, way too many insta-kills in that game. However, I love Dungeon Crawl: Stone Soup's difficulty because it feels fair, and deaths seem to happen based on my carelessness and lack of strategical planning.