"Continuity". That's the key word here. Star Trek consists almost entirely of stand-alone episodes -- there is, at best, a very thin plot throughout the show. Babylon 5, however, is essentially a really long mini-series. It is one continuous tale that needs to be watched from start to finish.
The story is essentially the tale of a collection of space faring races trying to not kill each other more than necessary, and the eventual arrival of a new and deadly force and the alliance that must come together to stop it. Nothing that hasn't been done before in countless sci-fi stories, but this is really done well, certainly the best of anything commited to film. The show is more militant than ST:TNG, being much more similar to the later seasons of ST:DS9 -- more good old fashioned blowing stuff up than we got out of Picard's ship. Also, while Star Trek tended to use models for their space scenes, which resulted in pretty limited space battles, B5 used computer graphics, so the space scenes of ship to ship combat are much longer, larger, more intense and more frequent.
If you enjoy a good sci-fi show, it would be criminal to not get Babylon 5. To me, as someone who loves Star Wars, Star Trek, Aliens, Starship Troopers and sci-fi in general, Babylon 5 is the single best show you can find. I only wish they would release the seasons to DVD faster, cause now I need season 3!