Above is a screenshot of what I see. The '.' in 'items.length' is hard to see. Semi-colons are not as difficult but look more like commas. The dot over the letter 'i' in the various words are also difficult to make out. I recall older versions of ACE had a much more readable font at 12px.
Using Firebug, I whittled away at the .ace_editor CSS. "Ubuntu Mono" on Windows 7 is the primary culprit, "Droid Sans Mono" is the secondary culprit, and "Consolas" is the third. The font selection only looked good when 'font-family' was just 'monospace' as seen above.
Based on this experience, I've come to the conclusion that there needs to be an API exposed at the editor level that lets the developer select the font set for the editor and the API needs to be clearly documented with a good example.
What api do you propose to add? I'd say one of the following is easy enough to do, but there might be a nicer way.
editor.container.style.fontFamily = "monospace"
Added fontFamily option for editor see https://github.com/ajaxorg/ace/pull/1304/files#L5R1429