There are dozens of "why root" threads in the D1 forum...you're best bet would be to look at those and remove one thing we can't do (yet) on the D2 when rooted:
- Install custom kernels to overclock (run CPU faster) our D2
Since we have a stock 1GHz CPU (compared to the stock 550 MHz CPU on the D1) overclocking is not a big deal.
Basically you don't lose anything by rooting, other than having an additional step if you ever need to return your phone to VZW under warranty (you have to unroot/get your phone back to stock - takes about 10 to 15 minutes).
You gain ability to control your phone...useful/fun if you care to, not everyone does. You can load custom ROMs (which attempt to improve the stock OS by removing "bloat" (in the eye of the beholder), or adding/changing functions/features, and use WiFi tether to provide an internet signal to a laptop if you're away from wifi and want to use your laptop on the internet.
Basically getting Root makes you commander of your phone, for good or evil. If you want to tweak/play w/stuff like that, and aren't afraid of some "lightly" technical stuff, it's fun. If you're happy w/the phone as-is and don't feel the desire to futz around, it's a waste of time to root, as you'll never use the capabilities.
Hope that helps - but check out the D1 "why root" threads for more info.
Also, there are only a few custom ROMs available for the D2 so far, so there aren't a ton of options out there like there are on the D1 side...
