I would also say fit is #1. But assuming that, other things I look for:
Non-super-stretchy neoprene.
From what I understand, that makes a suit easier to fit, but the neoprene is more compressible (plus may "thin out" more where it stretches to fit you?) and thus less warm. I look for a suit that fits without needing to be ultra stretchy.
My suit is "good" neoprene, from what I can tell -- it lies somewhere between the super-stretch, and the "firm" Rubatex that Jim mentions (I don't know that there are any non-custom suits offering the Rubatex now? I guess it would have to fit more to a "T" so maybe that is why.)
Double ankles.
My suit has an inner ankle cuff, then the main suit legs also come down and have zippers. I keep the cuff next to my ankle, then put my wetsuit booties on, and then pull the main suit cuff down over them and zip it closed. This makes for a great ankle seal. The inner cuff is thinner, stretchy neoprene, so while you do have to pull it over your heel, I don't find it arduous. Note that if you are a "pee in the wetsuit" person, then you would to do a bit more work in order to "flow through." (The same construction at the wrists is nice too.)
Back zip.
I find it harder to get out of front zip coveralls (presuming usual vertical zipper) because, while my arms can hang forward, they don't bed backward as well.
Pockets.
I agree that pockets can be a good feature. I didn't see any stock suits that had them (although there might be some), but you can pretty easily glue or sew pockets on afterward if you want them (and in so doing you can choose the pockets you want, which may be nicer anyway).
Knee protection.
At first I thought "Why does my suit need knee protection?" Then I kneeled on a moving boat deck (which is often grippy/textured non-skid), and I was glad my suit had knee pads.
Grippy bits
The suit has some grippy texture on the the back side from the waist to the ankles. This can be nice anti-skid on a boat bench.
Neck zip.
This isn't something I particularly need, but my wetsuit has a gusseted mini-zipper at the front neck area. It's so you can get some "neck relief" when you are wearing the wetsuit but not diving (of course you can also open it underwater if you want to). I'm not a real "neck claustrophobia" person, but my dive buddy loves this feature.
Another thought on material: Some suits have a "skin" lining with a coating. I haven't used one of those. I suppose you need to look out for abrasion/wearing of the coating. Mine has a sort of "fuzzy" lining. It's nice but I'm sure takes longer to dry.
Back in my whitewater days, we used "skin" faced neoprene and that was really warm and sort of "clung" to one's skin. The coated ones mentioned above might too. If I fit into men's sizes, I think I would try one of the freediving suits like Mako sells. I think they may be a bit more delicate, but they are supposed to be really warm for their weight/thickness. Based on how the "old fashioned" neoprene fit, I can see where it might be really nice.
A hood or hooded vest can help to extend, and might make a 5mm suit work for your temps, albeit perhaps not for multi-dive/multi day trips with cooler air temps. I have only dived mine in 66º to 68º temps. It was *just* okay. But I'm probably a bit of a chilly sort.
