Nothing is terrible, I suppose, but in the web client I personally enjoy the fast search, filtering, labeling, split pane smart inboxes, integration with Calendar, modal keyboard shortcuts (they aren't operative when you're typing in a text box but are active otherwise) and non-intrusive threading (conversation-view).

The fact that all this is the same on my Macbook Pro, on my Arch Linux desktop, or when I reboot to Windows, without any effort on my part to synchronize settings makes this all rather excellent.