I rooted my D1, main motivators were:
1) Overclocking (550mhz native boosted to up to 1.25Ghz). I run 1Ghz, phone works great w/ no complaints / excessive heat / battery issues.
2) Custom ROM (improved speed, features, style, custom themes, and much more). I'm running Simply Stunning 4.8 (dev ChevyNo1). Stable, fast, blue theme, custom icons, more custom details; highly recommended. No issues installing ROM
You can save an image, and restore it, in case you hose something.
3) Ease of change. Re-design your UI at will, with any number of options.
4) Unlimited access to phone to install whatever you wish, and restore your image if you hose it.
Its total freedom.
The D1 is ideal for lots of reasons, most importantly its just about as pure Android as you are going to get. No Motoblur, no Samsung UI, Sony UI, bloat, Vzware etc. This is why the D1 is so widely modded..
"Jesus has my heart, but Whiskey has my liver" --Jason Boland