Usually some very good snow, big domain
Chaotic roads on peak hours
The Soldeu /El Tarter resort is part of the Grandvalira which comprises a huge ski area where you can find runs for mostly all tastes. It is one of the biggest resorts in the Pirenees.
There are some very long, well groomed runs that are a real pleasure to ski and my favourite feature is that on some of them you have lots of trees surrounding the pistes. I really don't ski off-piste, so I cannot speak for it. As for the snow park, well, there are enough high profile international skiing/snowboarding competitions happening there for it to be a given that it is quite good.
Andorra is a charming little country, which has only one problem--the traffic is killer on peak times, when you're trying to get to the pistes in the morning or getting to one of the towns in the end of the ski day.
The aprés-ski is very good across the place.
All in all, highly recommended.