The toolbar is split between the tools section (for object tools, or mesh tools, or curve tools, etc.) and an operator section (the place where all the options for adding meshes appear). When resizing the split between the two sections, whenever you move the split to the very bottom, the operator section disappears. At first I had no way of getting it back, until I found the small plus sign on the bottom right corner of the 3d view. I would suggest putting this in a place that is more logical, such as at the bottom left, since coming up with a connection between those two very different places (the operator section of the toolbar, and the little plus sign on the bottom right) is very difficult.

Next, if I moved the split to the very top (instead of the very bottom), the operator section still disappears. Maybe this is by design, but I think that it would be much better for the tools section to disappear, since that is what I was trying to get rid of. Having the operator section disappear was very confusing (although by this time I understood how to put it back at least).

I'm not completely sure whether this was all by design or not, but I think it should be changed/fixed in order to get a better workflow.