It's not actually the touch screen experience - I can take it or leave it, and it was faster typing using the keyboard on my old blackberry (sigh!).
It's not the fact that you get access to hundreds of apps which may enrich your life - or drain hours from it - depending on what they do (watch out for osteopathy.org's app coming out next month, by the way!).
It's the fact that Google's operating system works so seamlessly with the basic programs like email and maps, that really thrills me.
Take the maps application: when you first use it to get driving directions, it tells you that speech isn't installed so you'll need to install it. OK, you think, I'll just press 'install' now and then it'll come in handy next time around.
Then a few miles into your journey, instead of going 'ding!' when you need to take a turn, it starts saying 'Turn left in 300 yards'. The speech module has installed and started working with the maps application! Maybe it's how it should be, but when you're used to computers you do not expect this sort of joined-up thinking!
Call me a Google fanboy, I don't care!