The radiation pattern is not affected whether you install it on a wall or a drop ceiling (facing down to the service area). However, the back of the AP has a metal plate that reduces the RF pattern in the back of the AP. If you are installing a multi-floor deployment you can reduce the RF that bleeds into the upper floor by installing the AP on the ceiling, otherwise you will have more bleed into the upper floor.
If I install it on the wall, the AP orientation is it critical? e.g if the 4 leds, on the AP enclosure are orientated towards north, south, east, west. Can I change the polarization or antenna performance?
It will not matter...you can't improve it and you can't make it better :-). If you need a different type of RF pattern (say for directional service down a corridor) you can use the 3715e or 3825e with one of the directional service antennas that we offer.