UI performance was great due to early use of ajax. If you had bad connection, which was extremely common in 2000's, then it worked way better than any alternative.
Of course, now it's the other way around, static websites are fast and SPAs can be dogshit slow.
it's immensely fast when compared to modern crapware-gmail.
and btw I remember hotmail too... the nicest thing was that you didn't have to refresh the whole page, e-mails just appeared (ajax probably).
and the conversation view, that was really innovative. i mean, mail clients had been doing threading for like forever, but wemail mostly didn't.
It does 'grouping' at maximum. Try reading a mailinglist in the gmail webinterface. A reply on a reply should be indented one level, not being shown at the same level as the mail it replies to.
Only in terms of storage and 'hive mind' spam detection. The UI was pretty shitty, and not much better than Hotmail.