The problems I have with the Freedom/Open system we have now are that it is not really free and open at all. You can not main heal as an Archer for example, you will only be effective at one thing as an Archer and that is Ranged DPS. The next problem is balance, I can see clear as day that this system is allowing me to create seriously overpowered Classes by allowing me to cherry pick the best abilities from all my Classes and combine them into a super class suited for a specific role. The next problem is that in an effort to achieve balance SE have made Classes very similar and this leads to the Class uniqueness problem. If SE place further restrictions on Cross Classing and make more things Class specific then what is the point in this open system.
I see nothing here that couldn't have been done better with the Job/Sub Job system of FFXI, only thing is that system allowed for more class/job uniqueness as it had tighter restrictions. If you were a WAR and you want to solo go /DNC, if you join a party and want DPS go /SAM, if you want to tank go /NIN. /NIN did kind of break the system for a lot of people but what do you think will happen when/if a skill like Utsusemi comes along in FFXIV that you are able to cross class? It won't matter what Class that skill is on or what rank it is, you will be expected to have it.
FFXIV with it's open system has the potential to be much worse than FFXI where elitism is concerned and a lot of people moaned about it in FFXI. In FFXIV with cross classing you will be expected to have those select few skill's/abilities that give your Class the edge when tough endgame roles around and certain builds start shining though. Only this time you might not have to level one sub job you don't want to, you might have to level five to cap just to get those skills to add to your super build just so you aren't considered gimp.