Well, with CV and CV VC and others it can vary hugely, but in my opinion with VCV:
-Standard settings, like ___ 450 -600 300 100 and such. Custom can work, but I never am comfortable with it and always have something to nitpick at. Well placed Standard otos make it very easy for me to go between VCV banks without a hitch.
-Good consonant speed, so I'm not missing your consonants but I don't have to apply Consonant Velocity, which I find makes things sound unnatural. I'd prefer not to use it.
-Recordings that have no noise glitches. I've been so sad when I'm preparing to use an UTAU for a UST only to find out that their "i ro" or whatever has a uneditable noise that makes them sound yucky.
-Flowing vowels. You have to do a bunch of editing for VCV banks that lack them, not fun. DX
-Stay on one pitch! It drives me crazy when in one string or in other strings they'll change notes. No! Unless you're pulling a crazy amazing Hibiki Shinji voice, it annoys me, and makes any Modulation usage impossible. (Shinji sounds horrible after only 10) And Shinji doesn't even really change pitches! ((name drop was to compliment Shinji and that only Shinji-esque UTAUs sound good like that))
These can sorta apply to all banks...
-ROMAJI ENCODED. It's not a must for me, but it's a must for many. And it makes finding files easier.
-Well sorted folders? Like for art, info, extra pitches, etc.
-Only .frq files to keep size down for downloading.
-Info in "Character" and "Readme"! It makes my life easier to have a site or a pronunciation for a kanji-named UTAU and other info.
-ZIP or RAR options for downloading an UTAU. I use UTAU ZIP because I want to, so I'm sad when one only comes in an UTAU installing package only. I don't use UTAU that way. And I'm sure many others don't as well.
-The name of your UTAU for the "character=___" or whatever it was. You'd think it's a no brainer, but I have five or six UTAUs literally named Renzokuon (in kanji of course) and I can barely tell them apart. i don't wanna rename them too much work too lazy
Ehhhh that's all I can think of. I been making sure to do all of this with my own UTAUs recently to make them satisfactory and "easy to use". Some seem a bit picky but I just think of them as common sense.