The one thing that can really be a problem, particularly when you are not that familiar with some area, is when you become interested in objects of some kind with a particular property and you do not know that these are usually being referred to as, say, Garfield-Dilbert-FooBars. More generally speaking, sometimes you do not know the name of a mathematical definition and that name alone would help immensely. I for one would like to have a hierachical database where you can find such names. For instance, you search for "surface", "complete" and "regular" and (among maybe other things), it tells you what a K3 Surface is. There should be plenty more examples like this.