With plasma there is more light "bleeding/mixing" to the cell next to it, because the mask is not as deep as LCD. An LCD pixel lies deeper in some kind of "tunnel", hence the worse viewing angle of LCD.
Some plasma panels have open cells and some have a waffle pattern....
If you imagine that on some plasma you have open patches of phosphor that is excited by a plasma discharge, but because these patches are open the plasma can excite areas around the pixel that should be illumined, this can smear the light and make a soft picture.
On some plasmas they use a waffle pattern that basically makes the plasma cell like a little cup, coated on the inside with the phosphor, this means that only the desired cell is illuminated but it is much more complicated to make, but the picture is brighter, sharper and longer lasting due to the increased surface area of the cell.
The fact that it is more complicated to make a waffle screen means that these manufacturers have more of a problem making a Full-HD set.
You should also be aware that due to LCD lag the effective resolution of a moving image on an LCD may be 1/4 of the signal resolution, a 1080 line signal may end up looking like a 260 line signal, this is why you will normally see slowly moving video demonstrated in shops.
If you want a good test try the end of the Matrix where there fighting in the rain, looks very poor on all LCDs and most plasmas.
The better way to look at it is that a plasma's picture is smoother rather than blurrier, when viewed from the correct distance obviously, which is why plasma looks very natural against LCD synthetic computer like look.