The problem with a great plot is, if your storytellers aren't so good, the plot can suffer. And at times, the bad acting made B5 a pain to me. Then again, Peter Jurasik and Andreas Katsulis (spelling???) made up for some of that quite well.
The Shadows really were nothing more then evil baddies for the sake of being evil. Unfortunatly, after a while, so was the Dominion. What started out as a great concept (a UFP but different, more of a dictatorship) quickly turned towards something more one-dimensional as wel. The idea was to have different races on the bridges of Dominion ships, all subject races to the Founders, making sure that by ruling them, these races could never hurt them. The Founders ruled the Dominion out of fear of being ruled themselves.
The Shadows.... well, some people just want to watch the world burn.
And that works great for a movie-villian (The Dark Knight), but as a basis for a serial villian that needs to work for two or three seasons... pretty difficult. JMS made that up by having the Shadows having bad-ass ships and plotting behind the scenes, but their overall motivation (aswell as the motivation for the Vorlons) was lacking, I feel. Perhaps I had hoped for something more grand.