Admittedly there are few inspections by customs officials, either with arrivals or departures. However, with the rate of corruption being as high as it is, there only needs to be one bad situation for you to find yourself having to pay large sums of money under the guise of a fine.
Drugs are among the items strictly prohibited to enter the country, and it would be better for your trip to Cambodia if you don't have any dealings with them. The country very strictly regulates the consumption and possession of drugs, especially with the locals. You can risk prison or very heavy fines. Drug trafficking is equally as prohibited and is also punished by heavy sentences in prison.
Firearms and dangerous materials will also be confiscated if customs officials find them in your baggage.
There are certain things that you should not bring back with you under any circumstances, among these being stones from the Angkor temples! André Malraux has been there before us and tried... and he was caught! The same can be said for the sculptures that can be found everywhere in the buildings around the old market in Siem Reap or Phnom Penh during your trip to Cambodia: certain items are not allowed to be taken away, but the lack of information can make authentication almost impossible. Officially, works of art or antique objects should be checked thoroughly to ensure that they don't form part of the country's national heritage. Sculptures of Buddha are often checked as they can sometimes be mixed in with protected pieces, and art trafficking is strictly forbidden.
Be aware, however, that inspections when leaving the airport are rare.
As in any country, there are limits on which goods you are allowed to take in your suitcase. You can take any goods with you that the EU itself authorises, for example:
Cigarettes limited to 200 per person; no more than one bottle of strong alcohol; perfume as long as it is for personal use.