As other reviewers have pointed out, the Angkor Wat complex is, indeed, awe-inspiring.
One of the moments for me, though, was taking a boat on Ton Le Sap lake - a truly massive lake upon which thousands of people live and work. The poverty is shocking, but who are we to judge.
The homestay overnight and following day's boat-trip and cycle ride on the Mekong were also inspirational, and have given me a few ideas about going back to the region and maybe exploring life on the Mekong in much greater depth.
The food! Everywhere we went.
The villagers and village life, and their friendliness in those parts of these countries. A complete contrast to the cities.
It is hot (>40C) in these countries in February/March, so people that have not done this sort of thing before might consider going a few days in advance to acclimatise. The back-up crews are always on-hand with water, electrolytes and fruit/chocolate, but make sure you take on water at every opportunity.
Try every type of food the guides suggest, including the crickets, beetles, and tarantulas to name a few. But, by the same token, check with them before you try anything radically new!