First, thank you all for the piece of gold called gnuplot!
During the last 7 years I am a happy user of gnuplot. To make it better I suggest to set the default line width and point size to value 2 -- usually, in physics journals the editors complain about lines too thin and point types being indistinguishable (say, not clear, if point type is open, or filled circle). Also, when redirecting output into ps or eps file, to make lines distinguishable, I always have to manually edit the final *.eps file for dashlength (default is 1, I put 3) and gnulinewidth, which is 5.0 (too thin, I put at least 7).
You might think that this is irrelevant, but believe me, when working on a paper, same figure file can be redrawn 40-60 times during the day to adjust the xrange, multiplot positions, etc... If done so, one additional operation (opening and editing eps file with text editor) is VERY annoying. It shouldn't be difficult to adjust it in the main code of gnuplot. Patching and/or recompiling the whole thing for personal use is another option I might go for, but this seems to be a general problem with lines too thin, so that many users have encountered similar problems, I think.