Agreed - if I remember correctly, the first user-facing added features were things like POP3, later IMAP, then google apps/domain integration which points at some serious back-end heavy lifting.
Now that the infrastructure is solid they need to continue to really drive and improve the user interface. It's still clunky with some searches and the interface can lag a little bit with the javascript.
Also, as I think about this, the themeing is also probably a step towards allowing Apps accounts to customize how the email looks for their domain and users. I hadn't considered that when I initially posted, and if that is correct that's a feature that will DEFINITELY help them push corporate accounts.
If I remember correctly, you can actually already apply some branding to a google apps account (if not for a free one then the paid-for version) at the moment, but I recall it being very limited. (just the logo, possibly)
(tangent: some bizarre modding going on here - I'd love to hear why)
We'll see how it goes though.