Usability 
The Old Reader seems to have been inspired by Google Reader before before Google removed all the social features. It lets you follow other user's feeds and easy share content with other Old Reader users and on Facebook. After Google Reader shut down The Old Reader was my favorite until I found InoReader.
The Old Reader is probably the most popular of the current crop of mobile friendly Web based readers and it got badly overloaded right after Google Reader shut down. It was slow and offline a lot at first but seems stable now.
The Old Reader uses a responsive design which works well in the Android browser but has a lot of wasted space at the top of the screen. It's usable in Opera Mini but some features like marking items as read are broken.