From a customer perspective - what's the most desirable / best characteristic of a successful startup vendor?

That they understand in detail the problem their solution is trying to fix.  And that their engineering works, rather than having a great marketing department.

 

And 2 extra points for not mentioning Gartner on their web site, PowerPoint or sales talks.

Humility. Thats a great attribute to have for a start-up organization. Most often you see they come in aggressively saying we have the X best solution, but that's from their perspective and may/may not be actually the case for each customer circumstances.

Ability to still exist as a company after facing challenges — both technical and economic.

Direct impact on pain point without a ton of additional layers of things I don’t need.

That honors his/her words. Because most of them sells something that their product/service can’t do.