Windows Mobile
Seriously. Why are manufacturers still making phones with Windows Mobile (6.1) on them? Before anyone starts, I'm speaking from experience. I've owned a couple of Windows Mobile phones. I'll never own another. This phone is clearly extremely good hardware-wise. Why besmirch it with a slow, buggy OS?
I could recommend other OSes, but that would just make this more of a flame. I recognise the whole "Office"/whatever applications/exchange thing, but come on, loads of phones do that now.