I think the most noteworthy feature is the 10G Nic which is more conducive to clustering than the standard GigE Nic for other instance sizes. I'm not aware of any other public IaaS provider offering 10G network currently.
In terms of raw performance the x5570 will probably perform very well relative to other IaaS providers. EC2 is one of the few IaaS clouds that utilizes a heterogenous hardware environment to enable better scalability between instance sizes. Other providers like Rackspace Cloud use homogenous environments (Opteron 2374 in the case of Rackspace) and rely on Hypervisor CPU throttling to enable CPU scaling between different instance sizes. On the low end homogenous clouds tend to offer better value but are not well suited for HPC application on the high end (e.g. a 2GB Rackspace Cloud server kills an EC2 m1.small, but the largest 16GB instance lags far behind EC2 m2.* instances). In fact in some homogenous clouds CPU performance is fairly constant from small to larger sized instances (you are essentially just paying for more memory).
I recently wrote a blog post comparing CPU performance of 20 different IaaS providers including EC2 and Rackspace Cloud. In it I used a compilation of 19 different CPU related benchmarks to produce an ECU metric for about 150 different cloud server configurations:
http://blog.cloudharmony.com/2010/05/what-is-ecu-cpu-benchma...