My spec preferences are for tonal purposes rather than functional or aesthetic bs, so big no-no's for me are poly or oil finishes. The former seems to always make the guitar sound choked, homogenous and stripped of lots of good tonal nuances and responsiveness in expressive in playing, while the latter seems to typically sound dry and lacking in harmonics and good connection between notes on leadwork. Ideally I seem to like shellac finish guitars, but since most vintage guitars use nitro I go with that as the next best finish option for tone. IMO it shouldn't be acceptable for guitars that are considered high quality to be using poly finish, but sadly most of them still use it
Other must not-have/tone sucker specs to me are Floyd or Kahlar bridges vs bridges that allow the strings to go through the body
Also I never heard a basswood guitar yet with remotely good tone nor an ash guitar I liked other than on teles and maybe a few Strats here and there. The best overall body tone wood imo for humbucker guitars is good Honduran mahogany (just hard to rival) and others I still have are alder, Spruce (most tonally complex wood ime), rosewood (yes whole body), Paduak, Poplar and would like some in the future in limba and port Orford cedar or Mediterranean cypress
Other than that not too many strong preferences. I like tall, fat frets. I like scalloped ones too. I like stainless steel frets also, but not a must have for me