Microsoft's previous platform, Windows Mobile, didn't have a marketplace until the very end of its life and that store never really took off. While it was never simple to measure the total app count for Windows Mobile, I doubt that even after a decade it was as high as what Windows Phone has achieved in one year. The Marketplace, with the apps all in one place, simplifies counting and drives a competition.
WP7 Applist tracks the number of applications for Windows Phone 7. The platform appears to be just a few weeks away from the 10,000 developer mark as well. Right now there are about 9,500 registered developers.
About 19,000 of the apps are free, while there are just over 10,000 that cost $0.99. This makes more than two-thirds of the apps under a buck. There aren't many apps between $1 and $4.99, but there are around 2,000 that cost more than $5. There are quite a few games at or above $5, so having a lot in that price range isn't surprising.
Speaking of games, they make up 16% of total apps available, with reference materials and books making up 15%. Users of both of those categories often purchase multiple apps, unlike other categories where you may only need one or two apps to fulfill a specific need.
The store is still well behind Apple's App Store, which has right around 500,000 active apps, and Android, with about 320,000. Both stores boast higher counts, but both have had more that 100,000 apps removed for various reasons, like not being able to work with the current version of the platform.
Microsoft has quite a way to go, but after one year, it has nothing to be ashamed of.