I run almost 150 mods right now but there are still plenty I won't use.
Town expansion mods are something I'm not really interested in, I've tried them in the past and they weren't my thing, I prefer the vanilla setup of cities/settlements. What I will use though are mods that redesign cities based on their concept art, most of these anyways, I haven't used any Vivec redesign mods because I'm worried about FPS hit/overkill.
Lore friendliness is something that I find really important. Don't get me wrong here, I love all kinds of mods, great work is great work, I'm just really picky about lore. For example, I don't use any landmass mods other than Tamriel Rebuilt and the Province Projects. I had to convince myself Lythidonia is cool enough to try out, exited for that. For the most part though the other reason I don't play with most landmass mods is that I spend so much time exploring Vvardenfell, Solsthiem and the Mainland I barely would have any time to explore anywhere else!
I don't usually play with mods that add new weapons either for the most part, I find there are plenty of weapons in vanilla that I don't usually bother modding more in. Would like to try out more of these in the future though.
Head mods have to be close to vanilla/close to what what represented in Skyrim/described in books/MK's concept art. I really really like MacKom's heads, and the topology and framework is so easy to use and make new heads with, it's something I've been really happy with.
Not sure what else to mention here, I think I mentioned most of it.