> You have to remember that when Gmail was launched it was considerably better than most desktop mail clients at the time.
It was not better for me than old FIDONet GoldEd 2.8x! It didn't (and doesn't) support proper threads, it supports effectively only top-quoting, it didn't (and doesn't) understand mailing lists in any way, it didn't support forward-as-attachment in both ways (now it supports it, though). Its filtering is still much more cumbersome than even Thunderbird one, not to say Sieve, and can be done only from web, but not from Android client. It doesn't support any crypto natively, both PGP or S/MIME.
Their are all features which I'm using daily (maybe, crypto is not daily, but still use sometimes).
Only good thing is labels, which is more flexible than tree folder structure. And, yes, full text search, obviously, as it is Google product.
As far as I remember, Thunderbird was not much worse than it is now, and it is supports most of this (though, quoting was and still is very weak, problem solved in FIDONet in 1990s!).
To be honest, I don't remember what was situation with address book and address autocompletion from it in Thundrbird before GMail, maybe there was none (but I will surprised, as, again, it worked in old TUI-based FIDONet client for DOS and OS/2), but this feature is trivial to implement in desktop app, and it could use LDAP or another centralized directory, not only addresses collected from your mail.
It was better than any web-mail, for sure, but better than desktop client? It is debatable.
1GB of storage is question of hosting, not client.
Update: Add to it modern gmail hate to self-hosting mail domains, and I could say that Google kills email as federated, free, non-vendor-dependent system. It is not surprise to me, of course, but still.