Caching, prefetching, and asynchronous operations hit the Hotmail web interface, making common email operations up to 10x faster. The NextWeb goes so far as to say:
No longer can we chide Hotmail for being horrifically slow. That is now Gmail's bag to carry.
I'm one of the few people on the entire team not working on performance for the last couple of months, but this is a huge win for everybody. If you've got Hotmail, go check it out.