First, the user interface is designed as if the programmers were incentivized to maximize the number of clicks required to get anywhere.
Second, it has the responsiveness of continental drift.
Third, editing and formatting text is an exercise in torture. When I want to delete text that I am writing, half of the time, the delete key won't work (I'm exaggerating, but not joking). Formatting of text is quasi-random. Want red-colored text? That works about 90% of the time for me. The other 10% will give me gray text (This time, not exaggerating). If you are brave, you can edit your text as raw HTML, but, my God, you'd better bring the anti-hypertension pills, because the HTML will blast you with a tsunami of elements. Sometimes the elements (unnecessarily) surround individual characters, sometimes they surround _parts_ of words.
Third, it is nigh impossible to set useful defaults. Why can't the due dates for assignments be defaulted to the end of the day instead of the current hour and minute? Do you honestly think that I would ever want my assignment to be due at 4:33 PM?
Fourth, it tries to do too many things. I already have email. I don't need Blackboard's email functionality getting in the way.
I could go on (for a while), but it's time for those blood pressure meds.