It really tends to come down to flow: is the spacing even? Are there uneven blobs of color where everything gets to seem too thick or thin? Do strokes feel like they narrow to join evenly on the 'm' or 'n'? Do characters like @, $ and %, the parentheses and quotation marks, complement the design, or have they been clearly borrowed in from another font, or not properly scaled to match the bold or light weights? How about accents?
This link shows you a font that deliberately breaks some of these rules (over-'constructed' characters, lumpy joins, a 'W' whose base is too narrow, etc). Tobias Frere Jones who lectures on type design at Yale has two good articles (part 1 & part 2) on mechanics of type design and how letters can look lumpy if misproportioned. Professor Indra Kupferschmid has a checklist, but it's not got any illustrations sadly. Paul Shaw's Flawed Typefaces article is very good at illustrating specific quirks in some very good typefaces that still might cause problems. Some are historically appropriate or justifiable in the name of variety, others were caused by long-gone technical limitations.