The term "plot device" comes to mind when we think about Georgina Sparks. She was brought back whenever the stories were falling apart, and it got old very quick. Georgina never changed. She was always looking for revenge, and it was embarrassing by the end.
On paper, the series finale was great. Chuck and Blair got married, and Dan and Serena got married. Shippers got their wishes, but the fundamental issue with the final installment was that Dan was revealed to be Gossip Girl. It made no sense, leaving several questions unanswered.