I'm using the standard CM fonts in 12pt (and amsmath if this is relevant). My problem is that
\Longrightarrow gives something like this output, which I find a bit annoying. I know that it's a composed symbol, but in 10pt and in 11pt it looks perfect, so why doesn't it also work in 12pt?
By the way, I produced the image by typing
\Huge\Longrightarrow on mathurl.com, so it's not an artefact of my TeX system (and it tells us that mathurl uses 12pt ...). Moreover, it's not just something on the screen, I also see it on a printout (where I don't know which LaTeX distribution was used for typesetting).
I think I've first seen this when I was still using LaTeX 2.09 a long time ago!