When we came back from Laos they dropped us off at a kind of pseudo bus station: more a petrol station/major crossroads than a true bus station. It was night at the time, and we didn't know when the bus for Siem Reap would be arriving. It's not that Stung Treng is any uglier than the other places: it just gives the impression of being laid out along four major roads.
The town serves as both a commercial zone and a transit zone between Cambodia and Laos, and perhaps eventually it will also serve as a tourist destination, if the Cambodians decide to introduce Mekong River cruises that is, which is certainly not the case at the present time!
In short, this is a town you simply pass through on your way elsewhere. It doesn't have much of interest to offer in my opinion. Don't spend much time here.