I miss 2nd gen and some of 3rd gen only. The answer is obvious: actual talent!!!
Normal length songs with actual structure, people who CAN sing and live at that, the groups looked excited and gave their all while performing, the concepts were different (even if freshly debuted from small companies yo could barely ever mistake one group for another) not so much ps and actually looked good, the dances were interesting and enjoyable, many of the members are actually pretty smart and witty so the variet aspect was cool. Just about everything. Now pretty much everything os robotised and uncanny valley and overly cultish nd fake.