I recently migrated from my venerable Palm Treo 700 to a Blackberry Storm II. In between I had a brief fling with a Droid, but jettisoned it after about a day. There were two problems. First, it's too much like having a laptop instead of a phone, IMHO. Second, I don't like the security model.
When we talk about the "Droid security model" we're really talking about the Android operating system and not about any particular phone. The exact phone I had isn't as important as the mechanisms that are undoubtedly common to all Droids.
The basic problem is that it's too vulnerable to malware like viruses, worms, or Trojan horses. This is a feature of its openness, but not a feature I personally crave on my cell phone. My phone serves a little as an electronic wallet, and I don't want malware in there, even if it limits my choice of apps.