It's important to keep in mind that these are literally what the title makes them out to be: simple signposts that indicate poor code might lie within.

I probably wouldn't immediately toss out a plugin that exhibited one or more of these signs, but I would take it as a sign that I ought to do a slightly more thorough skimming of the code before using.

It's a shame that the jQuery plugin directory doesn't make separating the good from the bad much easier.