The changes to the Gmail application include improved Priority Inbox support. We, ourselves, have not found Priority Inbox to be all that useful. However, there are quite a few apparently enamored with Priority Mail, according to Google's blog post on the Gmail changes.
However, Google also added the ability to email from any accounts you've set up in the desktop version of Gmail. That's something we've wanted for some time. Additionally, now you can easily switch between reply, reply all, and forward while composing a response.
Finally, you can now respond to emails in-line. All really cool stuff.
It's available in the Android Market today as version 2.3.2. The negative is that (you guessed it) it's only for Froyo (Android 2.2) and higher.