I'm coming from the Storm 2. Had the Tour before that and the Storm 1 before the Tour. I was a pretty big BB fan and still like the OS and the quality of the builds.
I'm a tinkerer. For me, it was simply a matter of becoming bored with BB. There's only so much customizing and tinkering you can do. I like Apps too and was less than impressed with the Apps I found for my BB. I can't believe how much better Facebook works on my Samsung Fascinate than on my BB. It also is more functional. Also, I was overwhelmed (in a good way) with all of the Apps available for the Droid.
Hardware. Much better, faster, and more modern. Very happy with the hardware in my Droid phone.
I do miss some of the customization on my Storm2. For example, setting custom text sounds for contacts. Believe it or not, I kinda miss the keyboard from my Storm 2. I didn't realize how used to Surepress I was. In fact, this is my only complaint with the Droid so far. I DO NOT like the keyboard. I hardly made any typos with the Storm 2. I make a ton with the Fascinate. I also hate how the Fascinate (Droid) will change "we" to "were" and stuff like that automatically. In fact, if anybody has any tips for typing or how to fix this keyboard, I would be in heaven!
All in all, very happy and pleased with my transition from BB to Droid!