The following is my PERSONAL opinion bout the disadvantages of an iphone 3G that i can think of now   :

-no physical keys & buttons (except the home & volume button)
-sucky camera
-sucky speakers
-bluetooth for headsets only
-no file browser on the phone?

Other prob can be solved by jailbreaking & installation of 3rd party soft...to heck with these disadvantages~ i still love my baby~   

Some basic features is not there,
eg:
1. SMS cannot forward msg, msg count.
2. No MMS
3. No AD2P (Copyright matters)
4. No Bluetooth file sharing  (Copyright matters)
5. Mono sound
6. Soft Sound Volume

But yet, there is alot advantage, eg easy install 3rd party software thru Cydia and Appstore which other phone is hardly competite it. Touchscreen is very very responsive, i do play with HTC diamond, u will feel no regret buy iPhone. Camera although is 2.0, but the quality is quite good compare to those camera 2.0. Still will choose iPhone in future .. Apple the best.