1. The handset is expensive.

2. Compare these prices with say T-mobile’s Flext 35 Web n Walk max tariff.

3. If you’re still determined to get an iPhone,

4. But once you have the money, you may end up having to queue like this.

5. When you do get it, you may find the phone's specification to be worse than the phone you replaced it with.

full list via maxpda

1. While iPhone will work with any SIM card from exclusive iPhone carrier AT&T Wireless No other carrier’s SIM phone will work with iPhone.
2. If you are wearing gloves, the iPhone’s virtual keypad won’t work. That’s a concern for wintertime phone calls in cold areas.
3. No memos.
4. No voice dialing.
5. No voice recording.
6. No games.
7. No capacity for video output to a tv (as iPod has).
8. No capacity for over-the-air calendar appointments.
9. No capacity for over-the-air software updates.
10. No IM. Gotta use text messaging.
11. No flash or zoom in the camera.

via zdnet

1. It costs too much

2. It doesn't fully support Exchange 

3. User-unfriendly battery

4. Where's the keyboard? 

5. No third-party applications

6. Stuck with one operator

7. The bleedingly slow EDGE

8. Smile! You're on iPhone camera

9. iTunes required to sync

10. The dead zone

via cnet