Image brightness/colour uniformity problem between bottom, center and top even when looked straight on:
http://www.digitalversus.com/duels.php?ty=6&ma1=48&mo1=141&p1=1562&ma2=36&mo2=402&p2=3707&ph=8
5ms response time compensation lacking TN monitors have actual average response time around 13-15ms while for IPS and VA monitors value is always close to real making them have less ghosting:
http://www.digitalversus.com/duels.php?ty=6&ma1=48&mo1=141&p1=1562&ma2=36&mo2=402&p2=3707&ph=1
(RTC equipped 2ms TNs are around 4ms)
I think the main reason people suggest TN panel monitors for serious gaming is due to the much lower input lag, some of the best TN monitors only have ~2ms compared to ~20ms for PVA / IPS monitors.
There was a link to a comparison chart in one of these threads showing different models and their input lag.