I was forced to leave Hotmail for Gmail because there has always been a problem affecting a significant amount of Hotmail accounts -- but not all Hotmail accounts are affected.
Basically, a large portion of mail simply never arrives. I'm not talking about POP clients deleting messages. I'm not talking about legitimate mail being sorted as "Junk." It just doesn't arrive at all. That includes a lot of activation emails from web forums.
I think I dealt with that for over a decade. Early on, I always blamed the systems that claimed to have sent the missing confirmation messages. I eventually figured out, without any doubt, that Hotmail was responsible. Microsoft / Hotmail support was no help whatsoever. Jumped ship to Gmail and never looked back.
I absolutely love how Gmail handles messages by grouping them into conversations. When I delete a conversation, all the messages in that conversation are deleted and I don't have to look through 5 folders to find the related messages.
I love how folders are really "labels," so I can mark the conversation / messages and have it show up in multiple places.
I absolutely love the instant search and the awesome flexible search syntax.
I even love the intuitive keyboard shortcuts (enable in Gmail settings) for mail handling. The most important functions (archive, delete) can be done with my left hand on the keyboard and my right hand on the mouse.