Problems with S5:
1. Some people always thought Babylon 5 was only about the Shadows and Vorlons. These people obviously were disappointed with everything that came after The Shadows and Vorlons left.
2. Some people hate the Telepath arc, and even though it only takes up a little more than half of the episodes in the first half of the Season, many people don't remember anything else about the season than just the Telepath Arc.
3. Season 4 ended with such a bang, it couldn't be topped. Also, as mentioned, JMS thought S4 was going to be the end, so, some of that non-stop plot explosion of the final founr episodes of S4 was meant to be stretched out through the first 3 or 4 episodes of S5, and the beginning S5 Arc of the Telepaths was meant to start weaving into the canvas in late S4, so, there would've been more of a smooth ride from Episode 4.22 to 5.01. Because of that cancelation threat, and tying some stuff up a few episodes early, and not starting stuff that should be started, you pretty much have S5 starting as a cold start and you have the Arc growing pains of early S1, which caused many people to get a bad taste in their mouths for S5.
So, in the first half of S5, I think you have probably about half of them are pretty damned good, and a few pretty decent and a few that are fairly average. but, the second half of S5 is as good as the best point in the Series, IMHO. And remember, a fairly average Babylon 5 episode is much better than an episode of many other Series, so "fairly average" is not really a bad thing in comparison to many other shows, it's just that when compared to itself, Babylon has so many awesome episodes, it's hard for it to compete with itself.