google's attempt at webmail.
- it's fast for the same reason google is fast...no banner/flash/etc ads
- 1 gb of space (at the time hotmail hadn't upgraded to 250mb/account so it was a bigger deal)
- you can search your email quickly with google's engine
- your email is parsed/checked for keywords so they can target you with specific ads (the ads are similar to the sponsored links on google)
Some people don't particularily mind that as anything private enough to not want a computer checking it for keywords will be pgp'd (or other) anyways, others don't much care for the concept on general principle.
The reason that they're invites is that they started it out as relatively closed beta testing, now however finding an invite on the web is like finding water in the ocean...but since it's still technically in testing it's still technically invite dependant for accounts.