Well, sticking with phones, the most commonly used functions are:
Voice calls
SMS
Contact management
Calender entry/notification
The rest is really just chrome, as very few people use it frequently.
With that in mind, why is it that so many phones have horrible contact management, SMS entry and calender management? I mean: Do they not do focus groups?
I am a researcher in the field of augmentative and alternative communications and mobile text entry (see
this paper (PDF) for in-depth coverage of the topic), and can tell you that it is well known that there are big problems with the most common T9 implementations. Even so, it is still today possible to pick up a brand new phone, where you have to start over fromt he beginning, if you try to enter a previously unknown word!
And don't get me started on phones with megabytes of memory, that will only let you store 50-100 old messages. WTF?!?!
So far the best UI for cells I have used is the SE one, as SMS entry is tolerable good and contact management can be done on the Mac using Address book. Still, I would love to see some obvious improvments in the UI, and - dare I dream? - better support for those of us who are bi-lingual in text entry! Yes, I know the last is a minority wish, but still...