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

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. 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. Best of luck!