Dan,
It's a difficult question to answer because each diver will have their own personal preferences.
Personally I would consider the following options:-
1) Kneepads on the suit because wear and tear around the knees can be a source of future leaks.
2) Sitech dump valve. I have used both types of Apeks valves and I think the Sitech valve seems to dump a lot quicker.
3) If you plan on doing lengthy time underwater then a P valve might be worthy of consideration.
4) For me personally I prefer a neoprene neck seal. A Latex seal is more waterproof but tighter around the neck than a neoprene seal. The reason I prefer a neoprene neck seal is that it's easier to get into / out of ... and the big thing for me is that I'm less likely to be sea sick with a
neoprene seal than a latex.
Having said all of this, the most important thing is the fit. A bad fitting drysuit is about as pleasant as a fart in an Astronauts space suit
As for bad things - can't answer in all honesty but a bad fitting drysuit is about ....
Edit: Just thought of something - Pockets. You need good sized pockets to store DSMB and any other bits and bats and 2 of them is a good idea but make sure they are fitted
on the side of the legs. Don't put pockets at the front of the legs - they are pain in the **** if you do any RIB diving.