For some reason, Google have never had a mobile-friendly version of the Play Store. Probably because Android devices have the Play Store app, which opens automatically if you tap on a link to the web site. The mobile site is a good idea though–users of other mobile platforms will be able to see what they’re missing.
The design is pretty much what you’d expect–it looks like the desktop Play Store, but optimised for small screens. It has a hamburger menu (albeit a more iOS-style one than an Android-style one), and you can select what device you want to install apps to. This will be helpful when you’re browsing on your phone, but want to install on your tablet (or vice-versa).
The mobile site is fully-featured, and even has access to the ‘Devices’ section, unlike the Android app.