One way to correct the wrong playcount is to open a previous Itunes library and adjust the playcounts manually.
You could load the old library (which in my case is pretty similar to the new one, since it is only 1 month old) and manually adjust the playcounts (by playing the songs an extra number of times) and also manually adjust newly added songs etc. Of course, you could also just use the old library to identify wrong play counts and adjust them in the new library.
How to load the old library? Hold down SHIFT while you open Itunes (first press SHIFT, keep holding it down, then open Itunes). Itunes will ask you which Itunes library you want to open, click “Choose Library”, then click the folder “Previous Itunes Libraries”. Within that folder you will find a number of old ITL files (ITL stands for Itunes Library File, these files contain metadata like playcount etc.), choose the one with the date before you updated your Itunes to version 11 (in my case the name of the file was “iTunes Library 2012-12-02”). Itunes will now open the old library with the old playcounts, song names, last played etc. You can go back to the new library with the same SHIFT procedure but then choose "iTunes Library" in the "iTunes" folder.
Comparing the playcounts of the old and new library can( for example) be done in Excel. Simply select all the songs in Itunes (CTRL A, CTRL C) and paste them in Excel (CTRL V), don’t forget to sort by name first.
Hope this helps.