Whereas Hotmail provided users with a chronological view of ever email they had received, Outlook.com groups together correspondents that have been sent by the same contact into an ongoing thread, making it difficult for users to pick out individual messages.
It's a similar system to the one employed by Gmail and the online version of Microsoft's corporate Outlook product, but if you're used to seeing your emails lined up in a linear fashion it may take a little while to get used to.