1) The iPhone OS will probably expire every couple of weeks. If you are out and need a phone the day it expires you are screwed. You will not be able to use the iPhone until you restore to the upgraded version. If the current version is leaked (meaning you are not a developer and got it through unofficial means), there is no guarantee you will have access to the next beta to keep the iPhone running.
2) The iPhone v3.0 OS is due out for this summer. That puts the release date to be about 3 months away still. That is a long time! The OS is not done. Why would you want to run a buggy incomplete OS on your iPhone?
3) The beta will have compatibility issues with some apps.
4) You will not be able to jailbreak your beta.
5) If you want some of the new features, read this post on how to use them on a 2.x.
6) Your 3g baseband will be updated, and you cannot go back. Yellowsn0w on 3g will also stop working. There will be no fix for this. It is permanent!
7) The 3.0 beta is likely very battery hungry and has a lot of debug symbols in use. It is likely also to use more memory than expected causing some apps to outright crash. Such is often the case with beta operating systems.
8) The 3.0 beta baseband is likely to not be very efficient and tested.
If you have already upgraded and regret the decision you find some instructions on how to revert to 2.2.1 in the post.