1st. Thumbs Up
a) Speed. Overall performance of the site is REALLY impressive. The whole open post/read/archive process is lightning fast. Flipping through messages is a breese. Plus, no un-needed graphic buttons/ads/crap, so everything loads very smoothly.
b) Interface. Finally a real exploitation of the DHTML in webmail! Address auto-completion is absolutely beautiful, the Spell Checker is really sweet (and keyboard controlled), the Grouped Conversation view is just perfect for keeping all corespondence relevant intact. A+
c) Size. Well. 1GB. Nough Said.
d) Ads. Can't beleive I'm voting for this, but... The AdSense in GMail is just awesome. It's probably the only type of advertisment I've seen on the web that I didn't add to my AdBlock list :) Having information directly linked to the mail I receive is an awesome advantage. And to those weirdos talking about gmail-is-too-creepy and etc... Open your eyes! Every mail you get is run through your ISP, or through Hotmail or any other webservice. The only reason nobody is talking about 'mails being screened' is because nobody knows! If the 'Big Brother' would want to read your mails... They would. So relax. The only way GMail is different from any other webmail in this sense, is that it visualizes the fact that mails are open ;)
e) Tools. Not really Gmail's 'fault', but... The geeks re-united and created lots of nifty tools. The main one I'm using is the GMail notifier extension for Firefox. Absolutely sweet.
f) Geekness. Gmail is uber-cool right now. The whole invitation thingie was a perfect publicity stunt (I think that was the main reason, not the beta-trial). Every geek on the planet has to catch a good name on GMail. Ph34r.
2nd. Thumbs Down
a) No HTML Mailing. Right now you can only create messages in plain text.
b) Un-Clear Spam checking. The way of spam recognition is very vague, plus when you select a mail as spam, it goes to 'Probable Spam' list, instead of being thrown into trash.
c) Annoying 'Delete'. Gmail's policy about the 'Delete' function is really obtrusive. "Never delete your mail. Archive, don't delete" and etc. It takes WAY too many steps to delete a mail. This is annoying.
d) Plain Filtering. Filtering system is VERY raw and VERY basic. What I would REALLY want to see here is implementation of something like an awesome open source project called POPFile, which uses Baysian learning system to automatically filter your mail, according to previous studing period. I've added this suggestion to GMail, but unfortunatly I seriously doubt that they will consider it.
Anyway, generally, I'm a happy camper and trying to move all my mail to my GMail account. Plus, having some invites, I'm starting to get my buds hooked aswell.
I sencirly hope that GMail will not stop at it's current state, and will utilize all the feedback from beta-testers to make it even better than now.