I've a few...
Nic-Strength - some companies don't offer a diverse enough amount of nic-strengths. I vape 12mg, and some companies do not offer it, just 8mg and 16mg. Sure, I could cut it myself, but I shouldn't have to. Sure, I could request a special order, but once again, I shouldn't have to.
Primer-less attys - this one is generally out of the hands of the vendor as most probably get theirs from China, but, I absolutely despise atty primer. So much so, that I will NOT purchase any that have it, no matter how cheap. Sure, I could get a jewelery cleaner and problem solved, but should I really have to? No.
Beta-test your new products and review them extensively before putting them on the market (i.e. Lookin' at you G4 cartomizers!) - Be honest with those who you sell to while doing this. If the product lacks in a certain criteria, TELL US ABOUT IT! Of course this is a bit suggestive, everyone's vaping experience is different from one another, but, if this is done, at least when we do shell out the $$$ to try the product, after it was extensively researched and reviewed by a vendor, at least we can make more specific and appropiate purchases based on, the very least, a vendor's review whom we might have similar interests. If I were a vendor, I would already be doing this.
I am sure there is more I am forgetting, and I agree whole-hearted with a lot of suggestions already made.