Gmail is just great. I seriously could not manage my daily emails anymore without the conversation view it offers, and of course the best thing is the powerful labeling/taging capability. Instead of moving your mails into specific folders, you just assign (one or more) labels to them, so one mail can show up in more than one of the virtual label folders. This is a great way to organize your mail, and I still struggle with the normal IMAP email I have at my new workplace, at least there are a few extensions, particularly GmailUI for Thunderbird that make the free mail program at least behave like gmail (it uses the same shortcuts with the extension and archiving is also done in the same way, although labels still are missing).
But back to gmail itself: There is even more to it, you can use it as your personal nerve center, doing things such as: Using gmail as a personal database, getting real-time news updates, bookmarking, calendar and todo lists, and even write blog entries via mail.
Read more: Turn Gmail into your personal nerve center [micropersuasion]