It's easier to design for scale-out than one might think; a great resource to get started is http://highscalability.com.
Regarding EBS, I haven't seen the hangup issue you've described. Any data?
As for low inter-node latencies, Amazon has an offering that specifically addresses that need: http://aws.amazon.com/ec2/hpc-applications/
Overall, I think Amazon is making a lot of inroads in areas with specific hardware demands. They just launched GPU compute options, and I expect we'll even see SSDs soon.
I'm the sysadmin (not the dba) for a website that does 30K http req/s at the edge, and our mysql cluster does about 3000 req/s during mid-day with most queries in the 1 - 5ms range.
Highscalability is a terrible place to get started, its a great place to share notes but if you get started there you will waste insane amounts of time on architecture astronaut nonsense.
I agree with nethergoat in that if you need low latency communication between ec2 instances that you should be using their hpc instances with faster/optomized ethernet.
I however disagree with nethergoat and agree with cagenut and think that the vast-majority of applications still currently depend more on a scale up paradigm. That is slowly moving towards scale out as people/businesses see its benefits and AWS is catering to this trend.
tl;dr - "price is in the same order of magnitude, but performance is two to three orders of magnitude different."