You definitely overestimate Petra's 2011. It's not like she was a 3-slam champion last year, consistently making semis or finals at all tournaments and ending the season with over 10,000 ranking points and three wins over a healthy Serena.
The fact is that in 2011, Petra reached the semifinals at just one of the 4 slams
and had very poor results at many other tournaments (IW, Miami, Nassau, Dubai, Beijing etc.). You also forget that she won Wimbledon without beating Serena, who had straight-setted her in 2010 and then showed her who's the boss again in 2012. BTW, Serena, coming from a serious injury, was beaten by Bartoli, who had been too strong for Petra in the Eastbourne final a week ago.
In addition, Petra was actually one of the weakest Players of the Year in recent years, winning just one slam and ending the year with less than 7,500 points, with her ranking significantly inflated by her season-ending 1,500 points from her YEC win in the absence of Serena and healthy Sharapova.
In fact, she needed three sets in both YEC semi and finals and her coach had to help her on court when she had blown or almost blown 5-0 leads in both matches.
This year, she has improved her year-on-year results at 3 out of 4 slams
and has improved big her US summer hardcourt record so saying that her level has dropped huge is simply not true.
Both in 2011 and 2012, Petra was far from being a dominant WTA player or a multi-slam champion and therefore there's a lot to look forward to as this talented young player works on her game and gets better.