I just got finished watching this series for the first time. It was enjoyable, with some memorable characters and performances, and an epic storyline, but I often felt like the characters suffered, taking a back seat to the plot. Londo and G'Kar were easily the most well-developed characters in the series, while others got seriously shortchanged. Sheridan rarely seemed like a genuine character; he came off more as a plot device, leading the series into whatever direction the story required. The buildup to his relationship with Delenn came off as very forced to me, which is a shame, because I know JMS intended to treat romantic relationships in a mature fashion on this show. Once they were
together, it was quite believable, but the process of getting there felt phony, somehow.
I'm also not a fan of Garibaldi being used by Bester to set up and capture Sheridan. The way Bester described his brainwashing was ambiguous enough to all but absolve Garibaldi of any responsibility for his actions. For a show that often dealt in shades of gray, it's interesting to me that
no one who opposed Sheridan's civil war was portrayed as credible or honorable, other than a few throwaway characters who lamented that it wasn't the job of the military to influence policy or initiate regime change. That came off more as hand-wringing than genuine conviction in opposition of Sheridan's agenda.
Don't get me wrong, I think Sheridan was right to fight Clark, but surely there must have been people who were legitimately suspicious of his motives and willing to step up against him on that basis, rather than just being fascist assholes who supported Clark's despotic rule. Garibaldi was the only person who expressed such misgivings with any elaboration, and he turned out to be brainwashed! The implication is troubling: "Only brainwashed dupes and slaughterers of civilians could oppose Sheridan!"
That said, I appreciated the slow burn of the early seasons, culminating in the Shadow War and the Earth Civil War. With Morden, you could tell Londo was biting off more than he could chew, and would only realize he was in too deep once it was too late. The price he paid in season 5 is a fine finale to his arc, as far as I'm concerned.
G'Kar's evolution was more subtle, as he was always a fairly philosophical guy, but he learned to become more thoughtful overall, even when it came to the Centauri. I appreciated his difficulty in getting other Narns to see his side of things without canonizing him--you could see all the hints of a violent, repressive religion coming out of the distortions of his writings. In that sense, I consider him a tragic character, as the lessons he intended to teach fell on deaf ears focused primarily on revenge and military superiority.
So, those were the two most interesting character arcs, to me. After that would have to be Kosh, who was often enigmatic, but through his actions demonstrated a deep affection for the younger races, and humans in particular. He was, after all, willing to reveal himself to save Sheridan, and died--twice!--in defense of Sheridan. For a character who spoke so little, you could sense the contrast between Kosh and Ulkesh almost immediately. It would not surprise me if Kosh felt some level of guilt over the manipulation of the younger races, specifically the creation of telepaths, given his attachment to Lyta.
Speaking of Lyta, after her return she was given some very strong motives, which continued to develop through the remainder of the series. You always knew why she was doing what she did, even when you didn't agree with it. Driven to desperate measures and all that. It was also interesting to watch her explore her abilities, which even she didn't know the full extent of. My only gripe is that we never got to see what became of her after she departed with G'Kar.
So, on to the characters that I don't think were as well-developed:
Ivanova -- Tough-as-nails CO with a soft side. We've seen it a million times before. Neither Christian's performance nor the details of her character's background made her very interesting to watch. It's a shame, because I think they could've gotten a very interesting character out of her, and we got hints of it from time to time, but she feels like a wasted opportunity to me.
Winters -- Really, what was the point of Talia, other than to stand in for Lyta until Tallman came back? Talk about shortchanged. Even JMS admits Winters was just a replacement for Tallman, and she became redundant as soon as Lyta became a permanent fixture again.
Franklin -- Other than his stim addiction storyline, which was alternately handled both very well and very badly, he was usually there just to play the role of the noble healer. The climax of his addiction storyline, which involved him facing up to a manifested version of his own demons, was surreal, to say the least. I appreciate the effort, but it just didn't work for me. It was just weird. It might be because B5 played it straight with just about everything else, so having any of those "mindfuck" elements show up really comes out of left field.
Delenn -- She was softened as a character by becoming half-human, but through the course of the series, her personality didn't change much. She was always even-handed, but willing to wreck tradition with abandon when she felt she was right. I get the feeling she existed primarily to "tame" Sheridan and be a symbol for human-Minbari cooperation.
Marcus -- Defined chiefly by his unrequited love for Ivanova, his death didn't have much impact on me. He added levity to dark situations, and it was nice to have a Ranger in the main cast, but did he really add much, or exist as a fully-defined character?
Vir -- For the thankless task of trying to talk Londo out of all the stupid things he ended up doing anyway, he got to become Emperor.

Enough said.
Lennier -- Another character whose principal motivation is unrequited love. Yikes. Is JMS trying to tell us something? His last-minute "betrayal" of Sheridan rang false in so many ways, I don't even know where to begin. It was like, "oh, shit, Lennier's supposed to betray the Rangers, how do we cram that into our penultimate script?!" I don't object to the notion itself, just the way it was done. It seems a bit cruel to assassinate his character at the last minute like that, and then not even have him in the series finale, so you don't know what came of it.
Garibaldi -- A decent guy with some serious problems, who was all but assassinated by that ridiculous Bester brainwashing storyline. I'm happy with where he ended up, but I feel like we wasted almost an entire season on things he did that he wasn't responsible for.
Lochley -- Not much to say here. A one-season replacement for Ivanova. The actress did a pretty good job given her limited screen time, but here is another person defined by their relationship with Sheridan.
Overall, I consider B5 a good series with some flaws. It's clear to me that the characters played second fiddle to the epic story JMS was telling. It stands in sharp contrast to DS9, which was usually character-focused and often kicked any ongoing plot concerns into the background if it meant telling a good character piece. I'm not sure which is better, since they had different goals. I personally
prefer DS9, but that's likely because my bias is toward character analysis rather than plot dissection.
Whew, that was a lot of typing. Sorry if I tl;dr'ed any of you.