I'm not into fonts, but reading anything on the web sucks after you get used to LaTeX.

  - Justification is not there and it just looks bad.
  - Paragraph width is arbitrary, which makes reading some emails (from folks who apparently think the earth is not only flat, but 1D) awful to read. I'm shown a 2000px+ wide, 60+ word line for a message.
  - Long words or non-English destroy line breaks and lines break at odd places.
  - There's widows and orphans around. I think I didn't even saw this one until I was told to fix my stuff during peer review, but now I see it everywhere and it only took a couple minutes to explain the issue and kind of ruin me.
  - Non-english keeps breaking the web.
    - Probably not just on typography, but many websites are still unable to deal with not so special characters like á, à, ä ø, £ and you get to read gibberish.