I've always felt the Toa Metru were sort of unsung heroes of Bionicle. The Toa Mata have nostalgia value, the Toa Inika paved the way for the final years of the line, but that leaves the Metru sort of behind, as the goofy angst-ridden protagonist of two pretty goofy films.
But looking at the sets, I realize why I loved them so much as a kid. The masks, to start off, invoke the same mythical feel the Mata masks had without the weird organic look of the Nuva masks. They truly looked like something pulled from a cyberpunk fantasy. Special props to the Matatu and the Rau.
Next, the level of detail in each element. Completely unmatched by any other wave's unique elements, the torso and armor and forearm/shins had a really intricate level of detail--tiny pistons and rivets that really made them feel like they belonged on the futuristic dystopia Metru Nui. The torso may have been gratuitously single-use and call to mind Knight's Kingdom or Galidor more than the constraction we're used to, but which torso did have a practical use? It was probably the worst torso in terms of practicality (what was up with the diagonal rotation?) but it definitely looked the nicest IMO.
And most importantly, the elements that introduced a new sort of articulation to Bionicle. It really started with the Rahkshi, but the Metru really introduced the ease of knee and elbow articulation through those double socket parts. Plus those pelvis connections. The Metru really took what the Rahkshi had right and refined it into a perfect format.
I really love the Metru, even though there were some funky design choices.
This blog entry is officially a Metru appreciation station.