This is one of the biggest concerns that a lot of us have about Final Fantasy XI right now.
First let me say, I feel like the recent changes over the last few months as far as enmity, and minor job adjustments have been steps in the right direction, but they are far too minor. There are still only a handful of jobs that get invitations to top tier encounters. It would be fantastic if there were class adjustments that are affective enough that open end game to all jobs.
In other words, I would love to see people shout for any "Tanks, Healers, Support, DD" Rather than specifics like "PLD, MNK, RNG, BRD, COR, WHM" etc..
Just as a couple examples: As long as Aegis and Ochain remain overpowered, then no other tanks will matter. I love playing Rune Fencer, but I'm basically relegated to soloing or lowmans because it's not as good as an Aegis/Ochain PLD. I also really like Puppetmaster, but my damage is murdered because of my pet constantly dying and the delays from slow job ability animations.
Boss evasion is absurdly high, and many DD jobs (especially pets) get annihilated by the damage. So you're only left with DD that can either survive the damage, or stay out of range of it.
It's stuff like this that really need to get fixed. The quality of life updates like the HP warps have been PURE WIN, but the game won't really feel fun unless we can actually be included in endgame while using our favorite jobs. I mean I'd say playing Rune Fencer and Puppetmaster feels like being picked last in gym class, but we're not even getting picked! ><
SIDE NOTE: Another complaint I have is with crafting and how it really creates a rather imposing barrier to end game progression for new players, but that's a topic for a different thread. I just wanted to mention it because I think class balance and old fashioned progress barriers are what are keeping this game from growing.