You have simple drawing tools available to you, you just don't know it yet.
To draw a straight line, click once anywhere in the image, hold ctrl.
To draw a circle, choose the round selection tool, right-click, choose "borders" and specify the size of the border. Same thing with a rectangle and whatever else you can draw with the selection tools.
Personally, I think GIMP could use simplifications in interface for some things; for instance, there's one tool to draw with soft edges, one tool to draw with hard edges, one tool for spraying, etc - These work in a very similar fashion, so why not combine all these tools to a single one with a few extra (visible) options instead? Things like, "scatter randomly", "blur edges" etc. Same thing with selection tools - is there any difference between round and square selection apart from the shape of the selection?
The biggest problem with the GIMP UI is that most features are hidden by default. I think Mark Shuttleworth is on to something when he wants to end the right click context menus, because quite frankly, they suck.