The famous blue screen, less bloated, open source and therefore a community support (look at the Ubuntu software centre, you name it and you have it for free) ....
I must admit that WP7 could (not necessary) bring in better UI and graphics compared to Maemo / Meego

Personally I don't want to use windows on my handset, need a bit of freedom or choices for lack of correct words from of M$ it already controls desktop to text editor to spreadsheet to presentation to project management to email to ....... thankfully I have switched away from outdated IE to FF or Opera...