I've taken a good hard look at the current filler arc (because I've grown weary of taking good hard looks at the manga) and in doing so, I've realized something.
It's fairly safe to say that the Bleach filler arcs have grown progressively better over time. The first filler arc, the Bounto arc, was, let's all admit it, terrible. It was ugly and unlikeable, and while most people would blame this on the producers trying to actually make it continuous with the storyline, I think I've found out what the problem really was.
It's the phrasing.
The Bounto arc introduced a ton of new phrases. Bounto, Doll, Bitto... it was all new and foreign. It seemed tacked-on to have these new ideas thrown around like they were nothing. It made the arc awkward and disjointed from reality, because these new things weren't things I really cared about.
The Shusuke Amagai arc was better, though not perfect. Realize, now, that this arc introduced only the phrase "bakkoto" to the mix. That was it. Nothing else was shiny and new, so I personally found I could relate to it better.
The Zanpakuto Unknown Tales arc was very good (by filler arc standards) and I think the reason for this was that there were no new phrases introduced. A few new concepts, like Muramasa's zanpakuto control, sure, but no new words to memorize or things to remember as we watched. That made it more likable than the two filler arcs before it.
The most recent filler arc, the Beast Swords arc, is just okay. Though I'd point my finger at the phrase "toju" and scream, I think I'll blame this one on its debilitating lack of plot.
I thought this pattern was worth pointing out at the very least.
By the way, does anyone else realize that, in combat, Muramasa would be virtually useless? If Koga were to go up to a Hollow and activate Muramasa's shikai, what then? What could he do?