I dunno, what some people would characterize as Bariel's boringness I would call 'serenity'. He seemed like a True Believer who actually had found peace through his religion...but that doesn't necessarily make for fun TV watching. And I agree that in regard to his relationship with Kira, it rather begs the question of how much it can really evolve. It might have been interesting to see Kira grow bored of Bariel's serenity though; perhaps Winn implements some sort of radical change (as per "Accession"), and Bariel's calmly accepting of it while Kira's increasingly upset by it, and by Bariel's unwillingness to take an active stand.