anyway, i think apple are aiming to have a good stable system at launch then gradually add more features. xbox live at it's start didn't have everything it does now, your comparison of the windows phones using xbox live is a bit of a stretch as really microsoft already had the infrastructure in place for it as xbox live has been up and running for years. now if you were comparing game center to xbox at it's original launch with the original xbox, you might have had a good basis for your comparison.
A more valid comparison is Sony.
Microsoft saw the importance of the online space and they made sure that certain hardware and software design choices were made to protect future growth of the Original Xbox, Xbox 360 and Xbox LIVE.
Sony on the other hand, really didn't consider some of these issues when they made the PS3. Things like Custom Soundtracks and cross-game chat worked from day one on the Xbox 360. But Sony didn't think about those features from day one, so they couldn't give developers the APIs to interact with those features. Sony added custom soundtracks with an update, but because older games didn't interact with the feature, it could only be used with new titles.
Apple would be faced with the same problem. Right now a developer must provide their own Achievement notification with Game Center (including sound). If Apple wanted to implement a standardized notification of its own at a later date, the whole point of doing that would be defeated. Some games would support it, others wouldn't. It couldn't possibly be standardized.
who's to say that game center can't do messages? there are api's about that let apps send messages from within the app, like vlingo composes the email in the app, it could use that for it?
That isn't a valid substitute for basic messages sent between people playing in Game Center games. Do you really want to give a stranger you met in matchmaking your phone number or email address? Then pay to send them messages?
Games are either not going to have any messaging system built in, or they will use their own system. Either way, it's not the foundation of a cohesive community.
as someone said, in the gamekit, there's an api for VOIP. Also, who's to say that apple won't add more API's over the next couple of months for an update in 4.2?
As I said above, if you don't do these things from the start, it's too late. The technical complexity of implementing such a basic feature becomes incredible when you are working with existing code.