I wanted to try windows phone few years back, but because there was a huge lack in apps, I didn't pull the trigger.
If you are accustomed to the sheer volume of choices that you would have from the Apple App store, or Google Play, then you will be disappointed, immensely. If you are new to it all, though, I doubt you will notice. There are some very good apps for Windows Phone, but a lot of the big developers are ridiculously slow in creating popular apps for the Windows Phone. So if you want every new Angry Birds game, too bad. If you want Instagram, you'll have to wait (it is currently in beta testing). And if you want an official YouTube app, talk to Google. But even with Apple and Google, volume doesn't mean everything. I used to have an Android phone, and a lot of the apps were crap. I'm not going to argue that there is less crap in the Windows app selection - there is plenty of it. Lots of people are trying to put out their own knockoffs of popular apps not yet available for Windows phone, and they usually suck. But there are some good ones.
If you are app hungry, don't get a Windows Phone. If you don't need a ton of apps, then you will find most of the really useful apps available, and some unique to Windows Phone that are actually quite good. I really like the Tiles format of the Windows Phone, and how I can put whatever I want on the Home Screen. I also like the Live Tiles, where you can see updated information without even having to open the app.
They are an acquired taste. They came to the game late, and have shot themselves in the foot in many ways, but I think they are continually improving, and I like what I am seeing. I like it enough that I think my next phone will also be a Windows Phone. Honestly, if I had an Android, the only other apps I would likely have would be games, and I don't like wasting my time so much with those any more. I have a tablet for that.