This is pretty much the only thing you'd be missing out on and it helps balance other content. I suggest if you are one of the people who farms gear from Toto Rak for Grand Company Seals that you do as much of it as you can while you can.
With putting a level cap on dungeons it'll force you to play the dungeon the way the devs balanced it. You may want to start considering other activities for Seal accrual such as Caravan Escort or higher level dungeons with a static group.
I personally will be welcoming level sync with open arms. It'll be nice to enjoy the lower level content again as it was meant to be experienced. This also encourages the development team to continue to release lower level content instead of end game content exclusively.
While there were gripes abound with level sync in Final Fantasy XI, this game isn't nearly as team grind centric as its predecessor. The level sync that applies to XI is a completely different animal than the one that applies to XIV. You can effectively solo experience in XIV while you couldn't so much in XI pre Crystalline Prophecy. You can also be power leveled with significant ease in XIV more so than in XI so the "people don't know how to do their jobs" argument doesn't really apply either unless you abolish the power leveling but even then, it seems that XIV is focusing more on event fights rather than exp. grind party mechanics so training in a trash mob grind situation doesn't apply to fighting in an event situation.
Level sync should have existed upon release of XIV considering the playstyle and demographic the dev team has been going for since XIV's release. I'm glad we're finally getting it.