She seems to blow it when she has good opportunities. The draw opens up due to upsets and/or she herself takes out a top seed, then she fails to follow up.
Her blowout loss to Cibulkova at the AO is one example. Serena was out, Radwa beat Azarenka and, while Li can certainly beat Radwa, Radwa has beat Li a few times, so it would have been a real opportunity.
People complain about the quick turnaround that she had to endure with one day between the Azarenka and Cibulkova matches. Sure, not ideal, but people act like she's the first player ever to have a tough match and then have to play the next day. That's ridiculous. It's happened throughout tennis history and players coming off a tough match the day before have gone on to win another match the next day. She tanked the match once she started off slow and she had a horrible attitude, thus ruining a decent chance to advance further.
She can always get blown off the court by an "on" hard hitter and her court sense and supposedly Mensa-level tennis IQ don't really have a chance to shine because she's constantly behind in the point.
Yet, she is very talented and can play fantastic tennis (see the Azarenka match), so if things fall right way and she plays well at the right time and has a good attitude, she could win a Slam.