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>What is it about the three neutrinos that makes them similar enough to each other and different enough from other leptons to all be called neutrinos?
charge = 0 is enough to distinguish from other leptons in this case. leptons are by definition neutral to the strong force, so neutrinos are unique in being neutral to both E&M and strong forces. If we found a new neutral lepton, somehow, I think the think we'd look for is if it mixes with the known neutrinos before we called it a neutrino
>And also are the three types of neutrinos more or less similar to one another than the 6 types of quarks are to one another?
I would extend that to say the 6 leptons are as similar to each other as the 6 quarks. In both cases you can write the weak isospin doublets (eg (u , d) (nu, e)). then there are the pmns vs ckm matrices, which are correspondent for the leptons vs quarks. I can see how this is kind of a semantic question, so someone may come in and disagree with me by looking at specific properties of individual quarks
