This is the main thing about Sentinels. They are not underpowered, they are simply complex and not easy to master. Many players become overwhelmed by the various abilities that have to be used in basic rotations and PvP situations. The problem is that Sentinel/Marauder's skill cap is REALLY high compared to the rest of the classes. The simplest and most noob friendly spec would be focus/rage and it is understandable why so many players would go down that path. Watchman and Combat rely on rotations composed by more diverse skills and do not revolve around 1 button.
Another trend I have noticed lately is ignorant players labeling Sentinels/Marauders as overpowered. The thing that they do not consider is the skill cap, which I mentioned. Skilled players really excel with the class and this is how it should be - them EARNING their way to being powerful due to adequate playing. Other classes have relatively low skill cap, especially BHs/Mercs, Sorcerers/Sages.