It's not perfect ... the browser is sort of "mobile plus". Meaning that you get a lot of the same feature levels you'd get in the mobile version of a web page (Gmail Mobile, Google Reader Mobile, etc.), just with touch screen enhancements. So, for example, the Gmail "Send as" feature, filter authoring/editing functions, label creation/editing features, etc. are all not present in the built-in Android Gmail client. Nor in the browser (oddly, the Android Gmail application isn't the same as the Android browser experience for Gmail).
And, no tethering support. But I hear that's more a carrier issue than an OS issue (they had tethering support in pre-release, but supposedly T-Mobile nixed it).
I'm not big on pocket media players ... though I did recently load the UMPCPortal podcast on my G1 and listen to it on my way to work, and liked that. But, that's the extent of my attempts to use the G1 as a media player. On Maemo, I sometimes used Rhapsody (but not often; I use it more on my desktop than on anything else). So, I'm not a good source of info about comparing them as media player platforms, nor am I going to be swayed in my preferences based on which one is a better media player.