Most lines should be 1px thick. For emphasis or for things like the outline of an IC in a pinout diagram, use a 2px line.
Arrowheads tend not to render properly in the PNG generated by the server. To avoid this, don't use any kind of arrowhead (normal, square, circle, etc.) and draw them in by hand.
The image below shows what happen when arrowheads are used. There are three groups of two lines. The top one in each group has an automatic arrowhead, and the bottom one has a very similar hand drawn arrowhead (arrow, circle and diamond).
As you can see the automatic arrowhead renders as a tiny off-centre smear, while the hand drawn ones render just fine. To see the automatic arrowhead as was intended, you have to go the SVG on the image page, and even then it may not work properly.
The problem described above has now been corrected in the MediaWiki software.