I think online is capable of delivering hard courses, not to is giving it a disservice.
The Udacity ones are definitely the easiest of the bunch. I think they aim to be. That's ok, it seems to be their market, at least at the moment.
Cousera courses seem to be more challenging, though I suspect they still fall somewhat short of the demands of the actual Stanford classes. But I get more out of them and prefer them to the Udacity classes.
The best class, by far, was the MITx Circuits class. I found it very demanding and felt a genuine sense of accomplishment when I completed it. Again, I'm sure it wasn't as tough as the actual MIT class since the exams were longer and open book, but it was by no stretch of the imagination easy.
If all the classes at the various options were like the ones that have completed thus far, I'd pick the EDx (formerly MITx) courses hands down.
Online education is going to take time to develop. If the courses are too difficult, very few people would make it through them. At this stage, they need the idea to catch on more than anything else. Once it catches on and they can establish legitimacy (i.e. being able to hand out degrees or find people jobs), then it'll be easier to convince prospective students to deal with more challenging material. It's a balancing act.