Level caps in general are needed for pvp balance, true, but that certainly isn't the only reason they're there. As to the question why 70/80/83, I imagine it's because that is the order in which caps were introduced originally. And given the insane amount of XP from 80 onwards, it seems like a reasonable milestone.I understood it that the lvl caps were introduced to even out PvP. In this respect, it matters not so much if some people people reach 83 a year before the slow lvling people (I guess).
Not that common I don't think (I have no numbers, though). It's still very well possible to kill him with 70s, but you do need more people, obviously. But again, whether or not Felankor is killable right from the start is really besides the point.I am sure 70 was pretty common, as KE had been out for a while when RoFD came (no Felankor before that, right?).