I'd say affordable hosting for the masses above everything else. If I can't play with it, I won't code for it.
However, with the advent of cheap VPS hosting, most of the people willing to try a new language/framework before it hits Drupal/Django/Rails momentum are perfectly capable of deploying sites. Everybody has a VPS nowadays, right?
Having a Heroku-equivalent or being built into shared hosting doesn't happen until you're nearly to critical mass.
(That said, I use nginx/unicorn)
But here's the thing: VPSes are so cheap now, I don't know why you'd ever go with shared hosting in this day and age. I don't have a job right now and I still haven't bothered to turn off my VPS, even though I don't use it... come to think of that, I should probably go log into the Rackspace console...