While it is true that some games are "comfy", it seems to me that it always varies from person to person. It's hard to really nail down what "comfy" is, since everyone is going to have a different opinion on it. What's comfy for one may not be comfy for another. There's no point trying to pinpoint the term.
So, instead of trying to nail down what the definition of "comfy" is in this regard, post an example of what you think is comfy and then elaborate on why you feel that way. Let me hear what fond memories these games inspire or why it's relaxing to play. Give me details, something to go on that can be discussed. I will start with my own example, it being VTMB. Usually I don't experience "comfiness", but this is the rare exception.
I like hanging around in this game because of how "alive" the cities are. Everything from architecture to the background music feels just right. There are billboards, flashing signs, people milling about, basically everything found in a city. The dialogue is very colorful and overall the game happens in a very believable setting, all things considered.
The "comfy" factor for me is the eternal night aspect of the game. You prowl the streets and talk to people always during the night, when everyone else is asleep. I've always enjoyed being around empty streets at night with nothing but the lamp-posts lighting the way. It's strange but also relaxing to see the usual busy streets with the people and the cars suddenly calm and empty. VTMB isn't exactly filled to the brim with civilians, so it's nice to sometimes walk around Hollywood or Downtown for example and see that there are only a few people around you. It's also fun going to the apartment and just idling, listening to the radio as I look at my latest e-mails while the TV is on in the background. The apartment has really good lighting, or at least lighting that suits my tastes. I also like hanging around the bars, especially the one found in Downtown. I sometimes pretend I'm sitting at a table there, so I jump onto a table and crouch in first-person. That is damned comfy (no pun intended).
Looking up NPC's skirts abusing the camera or staring at their asses is also a favorite past-time.
The game seems to give off this 90's feeling. I can't explain myself any better in this regard in that it feels like it is firmly rooted in the late 90's. TV is still big, computers and the internet are big news and hell, even the game's theme, which is vampires, seems to fit in with the 90's feel, since there were TV shows around the time concerning the supernatural. Everytime I play this game, it feels like going back in time, in a good sense.