6.0's universal change to 2-minute cooldowns for raid buffs has negatively impacted FFXIV's overall combat experience in dungeons.
DUNGEON LENGTH
Dungeon pulls typically take 1min/pack and 2min/boss, or ~5 minutes per checkpoint. With 2 minute CDs, every other pack now feels like a slog since all major cooldowns are still unavailable (2 usages/chkpt). In 5.x, there was usually always something up for each pack, either your buff or another's.
Bosses taking a bit over 2:00 does not feel good. Wasting your 2min CDs—now everyone's only real utility!—on a boss that will die halfway through Arcane Circle makes it feel that much worse. Jobs like RPR or DRG, which require time to properly get all their fun high-potency moves out every 2min, are now in a pickle since the boss will die before they can press everything. In 5.x, you knew Brotherhood would be back up in 90s, in time for the next mob pack. Now, it is simply wasted as that extra 30s means you will have to wait a full mob pack before you can press it again.
As another example, Trick Attack/Mug feels even worse with 2min CDs. If your dungeon run is taking on the longer side, every other pull you'll have to sit with Mug on cooldown waiting for a satisfying time to press it. This isn't about optimizing damage—it's about what feels good. Mug, Ley Lines, Blood Weapon—Universal 2min CDs forces more and more of these "not fun" buff usages onto the player.
TANKS
If you are a Tank, pressing your 1min buffs when there is nothing going on is not fun. The positive feedback telling you you're playing in sync with your party is now only every 2min, whereas before buffs were always rolling, with interactions that increased your damage—1min Trick, 90s Brotherhood, 2min Tech Step, 3min Litany—constantly rolling in and out.
Now, every other 1min window feels gutted. Not only are your own 2min CDs not up for these, but there is not even any compelling reason to do them, other than "It'll align things for the 2 minutes". This impacts high-end raiders looking for more dps, but also the casual player: it feels good to press buttons to get more damage. A 100k Hyosho Ranryu is more exciting than a 90k Hyosho Ranryu. Aligning with a buff makes you feel better at the game, with tangible feedback your party is actually a synergizing team. People like feeling powerful, and more buffs at different times more often makes each self-buff window feel better than a few 2min buffs.
There's no need to look at party composition anymore. Every DPS job has basically the same buff. Every Tank has the same rotation. Everything is same-y, and since we're running dungeons every week for tomes, helping this be the least monotonous as possible ought to be high on a dev's priority.
In short: not as fun! Things misalign and are less exciting with universal 2min buffs. Square please consider changing for 7.0 :(