I'm coming from Windows Mobile, not Android but that was a pretty open OS too (I still miss my HD2 sometimes, what a great Swiss knife).
I'm not too deep into Google services but I do have a Google account synced to my phone in addition to my Microsoft one (note that you don't need a Microsoft account to use WP) and everything works great, calendar included. Now if you have some sort of special relationship with Gmail, there's no Gmail app for you, just the native email app.
For the apps I'd say it probably varies from case to case and it depends on which ones you use regularly and which ones you can't live without. There's no official Dropbox app for instance but there is a Box one and a Skydrive one.
You should make a list of the ones you definitely need but I would say you're likely to find most of them nowadays on WP. Keep in mind that WP8 will run on Windows 8 kernel, which means in a few months the marketplace shouldn't be an issue anymore (in theory anyway).
Edit: Tethering just fine with my Lumia 900 on 3 PAYG.
Edited by Peew971 at 09:36:47 19-09-2012