Islamabad Airport has gotten a LOT better but still its not a first class capital Airport or as nice as compared to the KHI Airport. Some years ago it was a hole, looked like a crappy bus station, but somehow they have changed the outlook a bit by making new floors and some new adjustments, so its a bit better now. The first thing you get to see when you enter the airport if you are landing, is a big hall when you enter, yes you still need to catch the bus to go the the landing hall where the passport formalities and luggage handover. There are now new luggage carriers thank God, the 'Kulees' (Luggage carriers) are still there but they have controlled them now as they are not so pushing and demanding as they were before, they even wear brown uniform now.
The passport officers are ok, they sometimes take a pic of you, so dont forget to smile :-) There isnt any open rishwat anymore as before, some years ago, I was like a teenager and a bit behind my family, one fat & kala security officer asked me if I had bottles with me, I was like (§$"§$$$!!!) of course they meant alcohol. Now they are nice and dont ask openly at least, I think things are better in that direction.
Customs, coming in there isnt any custom, atleast I have not encountered any, going out, you have to go through the custom check, so make sure that you dont have anything thats not allowed through.
Check - ins, the most crappist part is when you are over weight, they ask for money for the over load, so make sure that you dont have more then you are allowed or you have some good 'connection' in the Airport. ;-)
Duty free shops, there are some, but the things are expensive, you would get much better deals outside in the city.
Thats all I can think off now. Hope it helped, btw are you going there?