Two key resources on usability are Rubin and Chisnell's Handbook of Usability Testing and Nielsen's Usability Engineering. While these methods are proposed for use with more generic websites, they would certainly provide a starting point. I would also consider incorporating ideas from Quality Matters and other resources on quality assessment in online learning. Know that many/most of the quality assessment systems out there are designed to assess direct instruction and do not conform well to collaborative or problem-based learning designs :( , so a usability tool for e-learning that encompasses more constructivist pedagogies would be helpful.
Utilizing the CogTool (http://cogtool.hcii.cs.cmu.edu/) may help to evaluate the UI design of your application and to compare it with alternative interfaces.
user-friendly interface, original content and Social Icons (with high rate of likes and visits)-last one will make you understand if you are on a right track:)