While their email service looks simple yet polished, their success will depend on how well they fight spam and the features that they offer (priority inbox etc)
I disagree, it will depend on their ability to put the product in front of users. The "techy" crowd might care about features, but few of the 500m+ (?) MSN users will.
Casual users like to ask power users for help. And non-techies can become picky for something that they use every day.