I think the mouseover text needs to be improved a bit somehow. For instance, when I mouse over UP in the paragraph above I find that part of the definition is cut off by my window even when it's maximized. I also find that sometimes when I mouse over the abbreviation either the bottom or top part of the definition is cut off (I was also having problems seeing the definitions at all at first, though I can't seem to notice this occurring now).
EDIT: Okay, so apparently I wasn't seeing the definitions because I was scrolled down too far.