As far as questing goes, one key difference between most of the MMOs mentioned and this one is, in this you are able to switch classes on the fly. The others you kind of just "respec" your current class to fine tune it (Which is kind of what you have with the action system here)

My point being is, these other MMOs have the luxury of you making a new character when you want to play a different class. Thus all the quests are reset and right from lvl 1 your off doing quest chains etc...

This game you can switch classes on the fly thus... you would need a different solution. Like what happens if you play through all of example: Blackshroud zones quests on one class? These would no longer be available to your other classes unless starting a new character. Granted there are a couple of ways to solve this if one was so inclined but it does add an additional challenge to design I think.

1. Add so many quests that you can barely see your mini map so you can have enough to support all the classes that you may play in the future.

2. Have so many repeatable quests that you wouldn't get bored from doing them all.

3. Create quest lines for each class per area that branch out and merge lets say mid 30s and could run each class through 1-50 (in addition to any future classes /jobs) With this comes having to implement a way for "you changing classes" to flag a trigger that tells the system that you are now on a new class so please show these quests and hide the others... and then you run into the problem of how do you solve the journal if there are quests over laps / names are the same... Do you make it so the journal is different per class? if so then will this effects leves? If not then do you solve this by having all quests with unique names? etc... etc...

4. Say that they only put in the enough quests to support 1 class after which you have to find alternative ways of leveling etc...

Then obviously there is prob other creative ways of making something like this work that I'm not thinking of... but to me currently in my mind... you would have to put a huge amount of work in to make questing a viable option for "level grinding". Anyway just my 2 cents.