OS-wide plugin support would be nice, so you can have things like a text expander that automatically changes a pre-set code that you type anywhere into a long phrase like an address.
You most likely wan't a smartphone that is easy to use without a major learning curve, and is a system that is fun to use and not very annoying. In that regard, the iPhone would be the best, due to it being the simplest and most fun current phone on the market, followed by BlackBerry, then Palm.
Unless you have Parallels running as root, you would get a request for password from OS X when Windows tries to do anything funny with the libary or the system, if you are a LUA then also with the applications folder, and some places in your home folder. so, unless you just type in your password in the dialouge box then click OK without looking, it should be OK.