A post by Tim Bray led me to a post by Mrgan whose main complaint is that you can't drag and drop files onto Gmail attachments.
I agree this is a shortcoming. But if you use Firefox you can fix it with the dragdropupload add-on.
And the advantage is that this add-on works with any web site/app, not just Gmail. Whereas, if one desktop app implemented drag and drop, that wouldn't help any other desktop app.
Mrgan's other main comment is that off-line already works with desktop mail apps. That's true, but I still think Gmail's new offline mode is better - you can use it on multiple machines, and it has a flaky connection mode. And unlike POP (IMAP is better), it keeps your email on the server so you don't have to worry about backing it up.
Even Tim Bray seems to think that email is better as a desktop app. Maybe that's because he uses a single machine (a laptop) for work, home, and travelling. For me, using multiple computers (and operating systems), I think Gmail is a lot better. I think I'd prefer it even if I used a single computer, but the advantage might not be as clear.
And, unlike Tim, I do think that Gmail's "conversations" are a big improvement. They take a little getting used to, so anyone who doesn't use Gmail won't necessarily see the advantage. But I find I really miss Gmail's conversations when I use other email apps.
Another thing I think Gmail should be applauded for is that it doesn't add any "junk" to my messages. I think it's pretty ugly when I get professional business communications with Microsoft or Yahoo ads (spam!) on the end of their emails.