1) Contributors have not been vetted - Some responses are based on real world experience, and some is conjecture from arm-chair quarterbacks. (A simple example would be that nobody has mentioned with any of the SuperMicro 2U Twins that you have to be cautious about the PDU models and outlet locations of 0U PDU's to not block node service/replacement in the rack)
2) There are multiple ways to skin a cat - There are many viable solutions in this thread, but you can't simply take a little bit of this, a little bit of that, and piece together a new platform that "just works." -- Better to go with a know working solution than a little of this and a little of that. Multiple drivers tend to be less effective.
I am the owner of a bespoke Infrastructure as a Service provider that delivers solutions to sites of similar metrics, and speak with plenty of real-world experience.
Larger Providers - We find a lot of clients move away from AWS, Softlayer, Rackspace, et al. as the larger providers aren't nearly as interested in working with the less-than-standard configurations. They want highly-repeatable business, not one-off solutions.
I'd love to talk in more depth with you about how we can deliver exactly what you need based on years of experience in delivering highly customized solutions. We'll save you money and headache.