All of the packages offer the basics when it comes to using predefined shapes such as rectangles and ovals, along with lines and curves. You might, for example, use these tools to draw a cartoon character with a perfectly circular head and eyes by using the Ellipse tool, or you might add a straight horizon by using the Line tool. As we’ve already mentioned, Krita features some fairly decent vector drawing tools, and these exist on a specialised layer and continue to be editable after they’re drawn. However, the two dedicated vector drawing packages (Inkscape and Karbon) are, naturally, more suited to images that are principally constructed from elements of that sort.
If it’s illustration-style image creation that you’re interested in, for things like comic characters and project logos, Inkscape is hard to beat, but learning how to use it can be a challenge. Once you’ve placed a basic shape such as a rectangle on the canvas, you can begin to alter every attribute of that object such as its colour, the line style and the bevel of the corners. Beyond that, you can edit the shape itself by, for example, adding additional points to an existing line or curve that, itself, can make up part of a larger shape. The downside of this level of control is that we’d doubt that any newcomer could get into detailed editing without searching the documentation first. From the developers point of view, it’s difficult to implement that level of control without increasing the level of complexity of the application and its interface.