Everyday we learn something new too and this was one of those things either we knew and forgot or just never knew, but evidently you can only have 15 apps on your device with push notifications.
Mind you, push notifications are those things we barely have right now but they do drive "live tiles" to update. And seeing that there are maybe seven apps that we know of that do push updates (Flixster doesn't count as they can't figure out how push updates work), we don't see this as an immediate problem.
But still, we imagine six months from now, a few of you will have 15 apps pushing notifications (and getting 2 hours of battery life--kidding) who will want to install that 16th app. Well, when you try, you'll be met with a "InvalidOperationException(Channel quota exceeded)" error. Of course, developers can code around this by letting the user know they should uninstall one of their other apps, but still...kind of an odd limitation. Though we suppose nothing is infinite, right?
Luckily, Microsoft has some time to push an update to change this should it become a problem, though we're not convinced yet that it ever will be for normal users.
Update: Developer Dave Amenta lets us know the following: "I have lots of tiles updating and there appears to be literally ZERO impact on battery (one updates at least every 10min)". So maybe we do want 16 apps pushing? ;-)
Update 2: Microsoft's Jamie Rodriguez responds. He notes that MS could change the limit in the future and they are constantly evaluating userr habits and needs. Doesn't contradict previous information.