The most legible typefaces are those that tend to be transparent to the reader. That is, they present the information in a clear, concise manner and call no attention to themselves. Highly legible typefaces are those like Century Schoolbook, Adobe Garamond, or Univers. Generally, the most legible typefaces are those that offer big features and have restrained design characteristics. While these attributes may seem contradictory, actually they're not.
Open counters help to define characters.
A byproduct of such open counters is usually a large x-height. As long as it's not excessively large, this can also tend to improve typeface legibility. Over 95 percent of the letters we read are lowercase composition. Within sensible limits, the larger the proportions of the lowercase characters, the more legible type letters are. Taken to the extreme, however, the opposite effect can result.