But there is a good amount of serious anime out there, you just have to look. I haven't watched many anime series from start to finish myself, but of what I have watched, Code Geass and Cowboy Bebop are great, and serious. There's a lot more comic relief in both of them than there was in Death Note but they're still serious.
Well, Code Geass DOES have a lot of comedy later on, but I don't think it was intentional
But yes, there is a lot of anime with serious storylines, you shouldn't judge all anime based on shows like Bleach and Naruto.
You know what I'd like to see out of a filler arc in an anime, if they're going to force it on us? A story separate from what should be happening in the main storyline. Just have it in the same setting and create some new, fresh characters with their own story to tell. Maybe explore existing character's back stories. Anything but this garbage...
I'd prefer if it just didn't have filler arcs at all, still, if the anime is getting ahead of the manga and you don't want to do what the first Full Metal Alchemist anime did and write your own original story that goes in a very different direction from the anime, that could actually be an interesting idea of what to do with your filler arc, they could use that chance to explore what's going on elsewhere in the setting, or maybe some of the secondary characters and other such things like that which are usually overshadowed by the main characters.