I really genuinely don't understand what you thought was so forgettable about Serenity, especially given Hades apt reminder concerning so many recent sci-fi films with pod all in their favour.
You had a raft of characters. Some with a few twists and curves.
You had plenty of chases, explosions, and fights involving weapons from sword to lasers.
You had screaming wild barbarians.
You had a surprisingly interesting bad guy who believed he was good. Indeed who some would still say was good.
You had some first class special effects.
A story that was easy to follow, but had a couple of minor twists.
So we can debate first place for film of 2005, but completely forgettable? :D