Paul Thurrott does a nice job of breaking down how calendars work on Windows Phone 7, specifically the trade off between simplicity and ability.
Windows Phone 7, via Live and/or Exchange, can sync up your calendar events. You can also add external sources like Google to the mix. All of these can by synced to the phone is a very easy fashion, the problem though is syncing back as you can only have one "source" that gets synced too.
Then there is the issue of multiple, nested calendars that are "subscribed" too within a calendar e.g. 'weather' within Google calendar--those don't get synced.
Granted, this is v1.0 of calendar support in WP7, but obviously some "power users" will have a tough time swallowing those limitations, while for us with not much to do in our lives (ahem, raises hand) it won't be a big deal. As Thurrott notes:
...you can’t be both simple and full-featured. Microsoft has opted with Windows Phone, in v1 form anyway, for simple. This is arguably the right choice, but the limitations of this choice will appear in multiple places all over this system. And this is just one basic example.