Granted, this has never been as true of Windows as Mac OS, but a desktop GUI tends to have a lot of rules. You have various type of buttons, sliders, bars, lists, tabs, and very well-defined behaviour for every single one.
Web-based applications have never afforded such confidence. If I change "tab" with the current Web page, will it use JavaScript to hide the current "tab", or just load a new tab URL and lose everything on the current one? Does this element on the page implement right click? Am I supposed to be able to drag this or not, and can I just throw random objects from Explorer/Finder at it? Keyboard navigation is just generally awful. That's what we're all here for, no? ;-)
Gmail isn't bad, but its insistence of staying in a single page with no dialogs or manager windows leads to a rather confused and odd interface, and they keep trying to be too clever with strange little widgets that don't behave in any logical manner.
I always found Mac Outlook Express to be the best graphical mailer -- drag and drop implemented to perfection, scriptable with event handlers right down to altering low-level message details, dynamically colours quoted text in the reply window as you add and remove '>'s, rewrap etc. Lists the sizes of attachments. Didn't need to re-Base64 the entire message on every draft save. The program couldn't have been more Mac like if they tried.
IMAP was a bit iffy though which would put me off now.
The Web is just too ad hoc for me and screaming out for some sort of standard widget toolkit where sites all behave predictably and remember that the middle button on the mouse exists for a reason. And that the keyboard does more than enter text :-)