1) Mail.app's weak IMAP support which drives me crazy on a regular basis - that and those god-awful lozenge buttons (which I used a hack I found online to replace them)

2) When using expose, even though I have a decent processor and a quite a bit of memory, there is some serious lag at times when I use it to switch applications

3) Funny wifi security protocol support - When folks use WEP of a flavour my mac doesn't support, and when you're on the road you need to be able to hook into anything (knowing as I do that WEP is junk - but it's a work thing). Yes, I know oh Apple Engineers that it's not the RIGHT way to do things, but this one gets in my way more times than I would like.

4) Address book forces you to enter things in successive order. Although I have gotten used to it, I've met quite a few people who are converts who it drives absolutely crazy.

5) Please release an iPhone with no phone, just wifi and everything else as a widescreen iPod. I love my hipster PDA, but I want the functionality and Cingulars' network is weak where I am.