EMail

Electronic mail, the king of the killer net apps.

It's ubiquitous, so it's tempting to build on top of. But would it be better to replace it entirely?

See Eric Armstrong's What's wrong with email
be careful what you wish for: text messaging has replaced lots of email traffic, but the UX is so much worse

Organizing your email

why bother, if you have Search? Because Search doesn't work great on email, so you often have to browse an awfully long list of matches to find what you really want. So if that starts within a more targeted subset of threads, you're better off
being able to find a message by browsing a rough period of time is handy, and that's a lot easier if not overwhelmed with low-value messages
being able to delete old messages of low-value makes Search work better

Existing organization structures

Buckets I think I want

low-value: spammy stuff from vendors I use, etc.;
- useless emails from Twitter, Facebook (I should just delete those)
transactional: purchase order/delivery tracking, etc.
social: newsletters, forums
known contacts; only humans?
other/unknown
?

A way of categorizing types of email: old notes

spam - commercial, unsolicited. Seems to be growing out of bounds. You can usually recognize it pretty easily, and delete it in bunches. A bigger hassle if you rely on slow (dialup) connections. There are spam-filter services to help, but not free.
- an idea that might help: filter all incoming mail against your address book - any unknown-sender email addresses get dumped to a probable-spam folder. (You'd put this rule after your list-to-folder rules.) MsOutlook almost has this feature, but not quite.
personal spam. Email from individuals you haven't met yet. A big problem for online celebrities, not for me. Email clients should match against your address book and let you filter/file email from anyone not in that directory. MsOutlook does not appear to have this feature (you can match against lists of individuals, but I don't think you can have an ever-expanding AllUsers list).
quick exchanges - short email conversations that are usually disposable
(hmm, disposability and thread-length are really orthogonal)

Email monitoring/deliverability