Being able to archive stuff and find it again by searching, instead of having to organize old email into folders. I never realized this till Gmail, but folders were just a clumsy, manual precomputed search result. (So are most book indexes.)
Spam filter. Gmail has blocked 2000 spam in the last 30 days. Spam almost never makes it past that filter.<p>Second is the ability to easily manipulate large amounts of email. I mean, easily archive, make read, etc. any amount of email through search. I like to see when people follow me on twitter via email. I get an email everytime, which means there is a ton of email that I want to only look at once. Gmail allows me to go: Search:"Twitter", then "Select All", Archive. Now all that is out of my inbox.
1. Speed (I think it's faster than Outlook in almost all operations)
2. VIM like keyboard shortcuts<p>1 and 2 <i>combined</i> makes it a winner for me. :)
The "send to" feature using Gmail is my favorite. Combined with the ability to turn the email into a Google document it lets me research, write and format articles or web pages without ever leaving "the cloud".
I use Yahoo! Mail for historical reasons. I receive about 12-20 emails per day. Besides writing and reading emails I use Search - that's the only thing I really need. I also started using folders this year: the folder names are 2005, 2006, ... 2008 :) Current year emails are stored in inbox.
On average day I spend very little time on emails, though.<p>I have a Gmail account, too, but I haven't found any Gmail-specific features I would use. Conversations or labels are neat and my spouse adores them. As for me I just don't care as long as my search box is working.
I have to say it's the lab features. With all the cool addon features you can enable on the fly. Canned responses and Undo features is the top 2 I can't live without.
Unlimited email addresses for my Inbox with a pattern such as myid-*@yahoo.com (e.g. myid-ycombinator@yahoo.com). Great for spam protection when registering on little known websites, etc. This is provided in the Address Guard feature in Yahoo email.
I use <a href="http://www.fastmail.fm/mail/familyplans.html" rel="nofollow">http://www.fastmail.fm/mail/familyplans.html</a> because it allows me to trivially manage everything related to email for my entire family.
I'm addicted to the temporary passwords on yahoo mail. I use them all the time when signing up on random sites.<p>Has anyone else made the switch from this to gmail?