Well it's a bit of all of your points.
DDOS Attacks, yes,of course, since many admins ain't willing to pay for a ddos protection because the cost is high, kids with booters & haters who owns usually a server but protected most of the times, they start ddosing so the players of their servers won't leave. Then is coming to the overused features & concepts. If you check around the Private Section & Preview zone, what you're going to see is 50 new servers, all of them using the same features & closing after 2 weeks in the best occasion. (L2Awakening, L2Amelia, L2WarPath) and many more servers with the same client, players, owners etc. Then it's coming to the developers. Developers & staffers who luck from unique ideas and they are copying other servers' rates & features, they don't even balance or try to clean/fix errors. All of the new projects are full bugged. None really cares to fix any of the reported bugs, players are also stupid without patience and they don't care if the server got a serious problem or not, they just want to play like addicted drug users. That goes to the bad community, and last but not least is the corruption but I don't see many players care about corruption these days.