There is a lot of web sites and applications that are just plain mediocre. Mediocre in aesthetics or function, and often both.
And really there isn't much excuse for that. Releasing a mediocre product, and leaving it that way is either a sign of incompetence or laziness. Especially with all the resources and general to specific knowledge that is readily accessible..
The key isn't in knowing everything, it is in asking the right questions and then following and pulling that thread until the problem unravels.