Where to start...
~ WHM and /WHM that die in stupid places without Reraise.
"I died in the middle of Despot alley, can you come get me?"
"No."
~ Mages that stand in range of AoE, get hit, then stay there and don't move.
~ BLMs who nuke too big too early and pull a mob back into the other mages, thus getting them all slammed with an AoE.
~ RNG who stand next to the WHM, RDM, etc. and do the same thing.
~ People who /WHM and don't cast anything but Stoneskin or Blink (BRD/WHM that doesn't use MP, might as well be BRD/DRG)
~ RDM that /BLM for Omega (related, if you have fought Omega, you know what I mean)
~ Cookie-cutter SAM who insist on using Tachi: Gekko 100% of the time.
~ Cookie-cutter THF who insist on using Dancing Edge 100% of the time.
~ WAR 75 that do not have either Great Sword or Great Axe skilled up including quested WS.
~ People who refuse to level a job because they don't have a Peacock Charm.
~ People who have been to a place 100 times, and still don't know where they are going.
~ People who break planned out skillchains on big fights, thus destroying the Magic Burst (I had a wipe on Shen from this)
~ People who show up to Dynamis/Sea/Sky/Limbus/Nyzul/Salvage/etc. with no buffer, then beg for R3 when MP is at a premium.
R3 is a luxury, not a divine right.
~ PLD that thing it's cool do tank PLD/NIN with dual swords on endgame stuff.
~ The fact that TP Burning (while fun), has destroyed the skill set of so many players. In Bibiki Bay, you had to know your crap or you went down. Hard.
I could literally go on for days, but there are some points to ponder.
p.s. ~ I used my Windslicer in Level Sync parties quite a bit, but I wouldn't dare use it in merit parties.