Good challenging solo content is rare in mmos for one huge reason, balancing. Take something like soloing bounties for example. I imagine that there are probably 3-4 classes or spec that do extremely well compared to other ones. Why is this? Making challenging content would mean encounter tuning based on the strengths and weaknesses of at least one spec of every class, which would be a huge undertaking if you planned on doing more than 1 or 2 of them. Balancing around group content is easier because you don't have to take strengths and weaknesses into account as much, and you also have to consider tanks and healers.