I was always interested in the character of Delenn, for example, but it was a constant battle for me. Having to take her seriously meant filtering out big parts of Furlan's performance which I just found absolutely cringe worthy a lot of the time.
The interesting thing for me is that I CAN actually enjoy watching people who aren't what I consider good or great actors and enjoy myself anyway. Arnold is a great example of this, but he's got such good screen presence that I get a lot of fun out of his movies.
Furlan IMHO just doesn't have that. She basically lacks the skill AND the presence to pull off the Delenn part sufficiently.
My favorite was how, whenever Delenn had to be profound or wise or all holy-like, she'd just raise her voice to a high whisper, and that's it. Or when she had to be serious or bad-ass, just make her voice real deep (to the point where she would actually croak out words), and that's it, no body inflection whatsoever.
Delenn had some powerful WRITTEN moments. Some of that stuff would be mind-blowing television with a truly great actress.
A lot of B5 actors though do get by out of sheer earnestness, though. You can just see and hear that Boxleitner isn't a praticularly good actor, but he does it with such gusto that its hard not like him, sort of like some of the dodgy but fun action in the original star wars movie.