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.
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.