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I think what I liked the most about the Metru era (2004-2005) is that it linked the first years to the later ones, both in aesthetics and parts. You still get alot of usage of technic pieces, beams and other such construction parts, but also many new parts and things like the poseable toa heads. Same for masks, 2004 brings the flat Turaga/Mata masks together with the, at the time, new Metru style masks, with a different way to attach them. The Mata and Metru heads both allow both type of masks to be attached, and both types of heads exist on Toa and Matoran. 2004 sets make me feel like it's all one big universe. While I loved 2006 and 2007, those year's sets lack any 2001 masks, they lack Mata heads and while all 2007 sets were amazing, they are mostly void of old parts.
I love each of the years, but 2004 and 2005 respectively were connecting the old to the new.