Microsoft ranks a distant third in smartphone market share, so it needs to do everything it can to encourage developers to build for Windows Phone. However, it also needs to ensure that its app selection isn't filled with low-value crud -- and starting from today, it's apparently going to be a lot pickier about which apps are allowed in the Windows Store.
A blog post on Microsoft's Building Apps For Windows blog highlights the new approach:
Apps that can't be distinguished from other apps in the Store, have icons or titles that are too similar to other apps already in the Store, or don't properly represent the functionality of the app may be removed. We may also remove apps that do not offer unique content, creative value or utility. For example when there are many apps that do not provide differentiated value (e.g. many flashlight apps with the same look and feel and functionality), some may be removed from the Store.
In other words: don't bring your terrible Flappy Bird clones round here no more. Microsoft also says it will remove apps that have a price that's wildly out of sync with its perceived value, and "informational apps" like user guides that don't make it clear that this is their main function. That eliminates apps which offer tips for popular software such as games and rely on tricking people searching for the actual game. There's also a crackdown on deceptive keywords and apps that boast they are much better than rivals (another trick to try and appear in search results).
Microsoft's move to promote universal apps that run across both PCs and phones also makes such a move desirable. Downloading apps for Windows via the store still isn't the norm for most PC users, and an environment filled with low-grade titles isn't going to make it any more appealing.
How we are improving Windows Store app catalog [Building Apps For Windows ]