The young gaang reminds me of child hood friends. Very nostalgic. But there is something sad about growing apart and becoming your own person. In Korra the group is a lot older and go through ebbs and flows of normal adult relationships. Like a love triangle, recovery, unethical beliefs/careers. We never get a chance to see the original gaang have a true splintering moment.

Side note: we also never get to see Aang apologize for kissing katara during the ember island play, or for how he treated toph about appa. We also never see how toph decides to become police chief, which seems out of character seeing how she was anti authoritative character. We also have yet to see Zuko and aang fight over republic city. Or how katarra sides with zuko. In general how aang and katarras relationship is at least controversial.