Education 3811
Entertainment 3587
Photo 562
Music & Video 1012
Health & Fitness 867
Security 134
Tools 2387
Government 111
Shopping 179
Games 3657
Lifestyle 1292
Food & Dining 754
Business 708
Travel 1050
Social 523
Finance 360
Productivity 1154
Sports 816
News & Weather 1598
Books & Reference 3158
As you can see there are only 111 'government' apps for example. Since the list of 'top x apps' only contains 100 entries, you're pretty much guaranteed to be in the top 100 of government apps when you make one (it can be anything, an app for a political party, an app for polls, etc.). The same goes for 'shopping' and 'security' (and a little bit for finance, which has 360 apps).
Edit: here are the stats for the different 'games' subcategories:
Action 183
Adventure 72
Arcade 512
Card 167
Casino 73
Family 258
Kids 390
Music 27
Puzzle 1269
Racing 33
Role playing 48
Shooter 92
Simulation 54
Sports 71
Strategy 203
I'm betting there will be a fairly sharp increase in the daily submission rate for games once Unity3D makes deployment to the Windows Store available for everyone. According to the Unity guys, they currently have 'no information' on when that will happen, but since some of their selected partners already have this option I'm guessing it'll be a matter of just a few months or so.
The numbers get even more interesting when you move into some specific subcategories: according to my version (UK) of the Windows Store, there are currently ~230 action games, ~550 arcade games, ~180 card games, ~80 shooter games and ~60 simulation games on it.
I doubt the numbers for the US and other versions of the store are very different, since basically nobody will restrict themselves to only the UK market if the game is already in English.
Now, I don't know any statistics on how strongly users depend on using subcategories to find games, but I know that's what I'd do if I wanted to find an interesting game.