The main reason why I like Bionicle is the fact that everyone was so close to nature and the elements. When our heroes were on Mata Nui, being a Toa of an element, for example water, didn't simply mean that they had the power to control water, it was a decision of life: you were that element and that element was you. This wasn't true just for Toa, but for the Matoran and Turaga too. The predominance of such a stunning and overwhelming environment around every artificial creation by the Matoran gave a real sense of mystery, but also a strange feeling of joy to me. As the Toa arrived on the island, they knew just as much as we did about that place (so basically nothing), and discovering the secrets of this world, only to discover that you have a sensation that there's more than meets the eye, especially after those eerie journeys underground, sparked a lot of interest. The original nature of the Makuta, with its strange cryptic dialogs, and Mata Nui, at that time just a "stone in a temple", created a mystical aura around the island, and the legend itself was so simple yet so shrouded in darkness, and seemed so "far away" and at the same time so tangible.
It's a bit sad that as the years went on, this original feel was lost, but I enjoyed those later years too, for different reasons. At that point there was already so much that had been discovered, and the world had expanded so much, that it wasn't simply possible to retain that mystery that the island of Mata Nui had. The same atmosphere though came a bit back with 2009 and G2, which is why I generally tend to like those years more than most fans do.
When it comes to the sets, I would be lying if I told you that they were secondary to the atmosphere (after all, I did collect every single Bionicle set ever), but I always felt like they looked more like a supplement to the overall environment more than anything to me (even though it's totally the opposite).
Lastly, I like when things get explained logically, but at the same time there's a lot of room left for imagination and for our own picture, as a Lego theme is after all supposed to spark creativity as its primary mission which is why I find most of the questions in the Ask Greg category to be stupidly pointless . Unfortunately this got totally lost in G1 after a while, but I'm grateful that G2 avoided it, giving us outstanding and inspiring art work instead, to flash out Okoto ourselves.