It is almost a tradition that open source products have poor usability. The target audience has often made the tradeoff of cost vs usability and polish. For educational content though, this is largely an unacceptable tradeoff. Wrestling with the design or interface will quickly distract and alienate the learner.
Much progress has been made on this front over the past several years, with open source products matching and sometime eclipsing their shrink wrapped rivals.
This is an area where feedback from the field is particularly valuable. Developers will try, but come up short, in viewing their work from the perspective of a five year old.
Then again, knowing some of them, perhaps not. :)