[quote=@Ace of Hearts]
What exactly about ME2 makes it 'filler'?
[/quote]

The whole collectors plot really doesn't mean much of anything in the grand scheme of things, they are just another baddie faction being controlled by the reapers. Apart from the human colony abductions to build that ridiculous terminator they weren't posing any sort of threat to the universe that the was some how more pressing than the reapers. 

Mass Effect 2 has a Dark and funny script with a some cool characters, but the whole game is essentially a glorified side quest.