When deploying an AP you should take into account both coverage and capacity but in the other hand you should be aware of the medium, for example how high the SNR is.
Some other factors to take care of are:
RF pollution
Other nearby Wi-Fi networks , mainly if you are using 2.4 GHz band
Any other device that uses the same frequencie, Again mainly in 2.4 GHz band
You can always use a protocol analyzer and look for high retransmissions, hidden node problems. among others.
Hope it helps
Victor.