Raw performance doesn't matter to me much as long as I can handle enough concurrent requests, and serve as few as possible with Rails (and trust me, I've built and maintained a webapp handling 50k+ concurrent connections). A good N-tier architecture goes a long way. You'll want a solid HTTP cache, an easy path towards integrating a CDN (better if you can resell and my IT dept has fewer vendors to worry about), and of course memcached is useful.<p>Creating an easy deployment process can be difficult but you'll find a good solution. No doubt you'll have a decent web management console as well. Devote some time as early as possible to the non-functional parts of the platform (fault-tolerance, availability, security, customer service and relationships) and you'll be in a good place.<p>Best of luck!
Heroku deployment shell script is nice. Minus is that workers are too expensive -- you pay a full one even if you only use it rarely. Scaling solutions like SimpleWorker can't access their shared database, so you have to upgrade to a super expensive one or develop work-arounds.
Not trying to start a hype machine. Please let us know what you do and do not like about the existing Rails hosting providers out there and the offerings they have. Your input is key :)