I like to play a role, because I'm a huge hypocrite.
Thinking a bit more about it...I like very different CRPGs, but the two things all have in common is exploration and character development (finding new loot and watching those little numbers go up).
Turn based also kinda hides the stupidity of the ai better. In fps games it is easy to spot the patterns in the ai behaviour and often times they just blindly charge at you. Not only is it repeating and breaks the immersion a bit but it also forces me always to make a choise. Whether to try to ignore the ai issues and just play the way game seems to expect you to. Or just abuse the ai weaknesses to max just because it can make a challenging bit of gameplay super easy.
I also like turn based combat even though it is way too simple and stupid to admit I like it already a lot like it is :/.
I don't fully know since I don't like most games I play, let alone most RPGs. But most of all, I'd say it's the feeling of 'grand adventure', the feeling of journey, hardship, meeting characters, experiencing a story, choice & consequences. I could care less about character builds and skill progression. If I want to calculate skills and game systems, I could just do a bunch of excel sheet excercises. I also don't want to 'master a game'. I don't really gain anything from achievements, breaking systems, beating enemies on hard. I don't know, I feel like I have that enough in my real life already. I came to a realization that I don't really care all that much about systems, as long as a game can draw me in and I can soak in the atmosphere.