Incentive: easier whatever (try the Flickr demo, it is quite cool)
Who's going to develop: Yahoo (obviously) and (hopefully) a flock of hackers (if _why is playing with it you can be sure it will attract a lot of crazy people :)
I don't have enough kind words for Redmine http://redmine.org. I have been using it for some time and it is perfect as a simple bugtracker/SVN browser/wiki.
It's open source and built in Rails, and it's easy to tweak to suit your needs (via a plugin system) and perfect if you want to get your feet wet with Rails.
As for speed, I'm using it on a shared host with Passenger and it fast enough. Speed on a VPS is quite nice, and it flies on a local network.
For more advanced stuff, your best bet is Prototype.
Dojo, YUI (and ExtJS, not on your list) are good if you have a massive team of JavaScript developers and need standards and solid documentation, but are overkill for 90% of projects.
looks like you've made up your heart and now you're trying to make up your mind and/or wallet.
It may seem silly, but for me the main reason to choose a computer is what I call "the regret factor" (i.e: how much you'll miss the one you didn't choose)
No amount of processing power or future expandability can compensate just a single "I wish I had bought the other computer"
Would love to hear more about the business side of this, because this sounds like the ideal side project for natural born tinkerers!
It may be too soon in the project's lifetime for any insights, but if there's any wisdom nugget you can share it would be really nice :)