I think that the biggest problem to writers with fillers is that they can't really do ANYTHING interesting, because that would interfere with the main storyline. Naruto fillers have been almost complete crap with some decent eps and one fairly good arc (I think the Kaima-arc was pretty cool and enjoyed it). With Bleach I think that the fillers begun nicely, it was pretty interesting, but the "mystery" of Lilin/Ririn and Co. was just dissapointing (I mean the thing with Urahara and all, hopefully I'm not spoiling...)