One of my favorite acid tests is address completion. When you begin typing an e-mail address, your mail program should immediately show you the matching addresses and then dynamically constrain the list as you continue to type. Outlook does poorly on this test; you have to type CTRL-K to invoke the address book in a separate window. OS X's Mail does address completion in situ, just as I expect. So does Gmail. And here's the shocker: Gmail does it faster.
Gmail's spell checker is another amazing hack. When you invoke it from the message composer, misspelled words turn red. Click one and a list of choices drops down, ending with an Edit choice. Click Edit and the suspect word converts, inline, to an input box. When you're done correcting one or more words they merge back into the text.
This is very geeky stuff, I admit, but here are two important points to take away. First, as I've often said, intelligent use of browser-based technology can accomplish more than most people realize. You can't do everything -- not by a long shot -- but for many of the things that information workers routinely do, even ordinary Web UI is good enough. And now Gmail is proving that we don't have to settle for ordinary.
Second, Gmail's architecture is not limited to Web UI. Because it is protocol-driven, developers can create new tools that speak to the DataPack format. Many have done so already. My favorite spam filter, SpamBayes, isn't yet implemented for Gmail, but if that happens, I'll be sorely tempted to switch to Gmail full time.
So is Gmail a rich Internet application? Sure. Although that label most often applies to Java, .Net, and Flash clients, Gmail shows that Web clients can join the club too. But crucially, Gmail's architecture is open to other kinds of rich clients, too. It doesn't have to be a zero-sum game.