The major issues revolve largely around end-game content, such as tier 4 open RvR, fortress raids, and city sieges. Many players in the endgame are experiencing debilitating lag, targeting issues in RvR, crashes, and even the inability to log back on to their main characters. A recent crash even created a rollback situation for some characters, which means they lost everything they had gained over a certain period of time before the crash (from XP and renown to Darkpromise gear).
This is an unfortunate situation for Mythic because it seems to be overshadowing many of the great new features in 1.21, such as a new token system for RvR gear and enhanced keep siege options. The living guild interface received a major overhaul as well and many players were reporting that cast lag was nearly abolished before the other issues started cropping up.