Using Gmail with Mail, Google Calendar with iCal
Keyboard shortcuts: Mail has it's own keyboard shortcuts, which you may have to learn.

Alternate email addresses/multiple accounts: works fine. Don't know about plus-addresses. 

Priority inbox: this is not supported.
Labels work, as does flagging.
Public calendars work in iCal.

Syncing is very easy to set up - via IMAP in Mail, and in Preferences in iCal. In Lion it's even simpler. 

Yes, you add each account separately in mail. You can still visit them in any browser or device.