I literally JUST replied on another thread about this.
There are already FB and Twitter notifications in the Me tile, but MS needs to add 3rd party apps there.
As for holding the search button, it would make more sense for that to go to TellMe, not notifications. Holding the Home button would make more sense than the search button.
I've always wondered why they mapped Voice to the Windows Key. It just doesn't make any sense to me. I like the idea for Long Press of Windows going to Notifications and Long Press of Search going to Voice.
This one is a sorely missed features.
Come on Microsoft.. listen to users... We need notification center.
In the meantime go here and put in your 2 cents
Toasts unnoticeable when phone locked
The lack of a Notification Center isn't the only issue with Notifications. The whole system needs a complete revamp. From the poorly animated Toasts, to the Lack of a setting to turn off IM/SMS Toast Previews (a privacy issue, IMO), to the lack of Email Toasts (forcing you to go into the App to see where an email came from), to the lack of a global notification cache, to the poor lock screen notifications and pretty much useless badges on third party app tiles due to background task limitations.
PUSH and the Notification system was obviously something they left for the last minute, and didn't really devote much resources to with Mango.
My MAIN issue with them is speed, though. They are unusably slow for anything other than Email, SMS (both of those are pretty obvious). Even IMs are noticeably slower in the Mango IM client than third party clients on other platforms. Third Party apps from the same vendors all give push notifications that are a magnitude faster than the WP7 versions on other platforms.
I'm not sure what the issue is. But it seems the entire system is either bogged down or just badly engineered, period.