Microsoft is testing in these days the web mail system that will be the Hotmail's successor, code name Kahuna, and Paul Thurrot (as usual) has a good review of the system.
All the attention seems to be centered on the interface... Kahuna wants to be as more as possible similar to Outlook (and I think this is a good idea because Outlook interface is every day more a standard) and the work done is certainly good (as you can see below).
However, there's a clear fact that must be considered... in my opinion, one of the big lack of Hotmail is on performances.
Hotmail is always been a slow system, expecially under heavy loads (I've never used it a lot expecially for this problem). Ok for the new features and the new user interface (AJAX,DHTML,CSS etc.) but to be a real success Kahuna must absolutely have good performances. Actually I think that Gmail is the point of reference on this field... Gmail has a simple interface compared to Kahuna but performances are (for the moment) extremely good (I've about 400Mb on my mailbox and it's always fast on every operation).
This is the feature that absolutely I want on Kahuna... ok for the new UI, the new features, but if performances will be poor the system will be the same toys without a real professional usage.