Our project has around 15,000-20,000 man hours put into it (probably leaning towards the latter). This means if somehow we magically found $200,000 on the ground we would still be short changed. You would be looking at, at least 350K-500K for a fair days work (standard AU wage from working in a supermarket | split between two people), just to cover what has already been done and not what is still left to do.
It adds up really fast on the big projects, especially with so many people being involved. If you multiply this by all the UMS developers, you get into astronomical amounts. It is a very, very big gamble to invest this amount of time into a project with no guarentee of return, but we all have our choices to make in life. I have lost a lot of potential income because of my dedication to my project, I did this full time, I work at least 10 hours a day, usually closer to 16, it is very, very hard on my life, my relationships (partner, family etc) because even though I am at home, I am not spending quality time with them, instead working my !@# off on a very big gamble I took.
I would like to see more incentive (as you mentioned) for developers to invest that time into a well polished project, but I honestly do not know what the best way to go about it is. I have my thoughts of what I would like to see happen to my life, but it wouldn't suit everyone.
The world is changing, there are too many people, you need to advertise everything you do. That is what cover letters exist for, say if you wanted to apply for a job at Blizzard, would you honestly send your resume without a cover letter (even though Blizzard requires one) - a cover letter exists simply to advertise yourself in a different way from your resume.
When there are too many people involved you need a way to stand out. Those who will succeed will be the ones who think outside the box. You can't simply make a good project anymore (in anything) and have it succeed. Minecraft is a prime example, it succeeded because of the cult following on 4chan. 4chan advertised that game more than anyone else and 4chan created the success of Minecraft. You don't have to advertise your own project but someone else will have to (usually through word of mouth), otherwise your map will not get played.
Concerning your conclusion; I think Blizzard needs to get involved more with the community, more constant featuring etc (like a new feature every Friday morning). This community involvement I cannot stress enough.
I stay awake everynight until 2 am (10am Irvine time) either talking to Blizzard Reps or waiting for them. The nights where I have to wait are really really bad. Because I am not going to bed with my girl, I am staying up because we need help from Blizzard for our project, whether it be bugs, following up on previous conversations, developing future plans or even just having a chat. When no one shows up in the IRC, or you send an email off and don't get a response, even with everything that Blizzard has done for us it still hurts mate, it cuts deep.
We had one and a half years of silence from Blizzard as UMS Developers, we need that communication now, we need that support now, we need them to show that they care about us - that is the greatest motivation that any UMS developer will ever get. And trust me when I say they do care about UMS developers, they are just not the best at showing it.