I'm a causual so my review is far from complete. (And sorry if my english is bad btw).

I'm about to get level 30 with a mixed team (1 gladiator, 2 thaumaturge and an arcanist -as healer, i plan to do a scholar-)
The game is interesting so far, it can be a little cpu heavy.

I did the 4 first instances and the first primordial.
I find instances are really challenging so far (i mean, i read some boxers tells there were easy, but every times i struggled to finish them).

For some quest on the story line, you have to disband and do them character by character, but i don't think it's a real problem. Remember on Wow all the quests you had to grab some items for each characters ? (Specially prior than MOP). All others quests are doable whith a group.
A small problem can arise with missions that you get with mandate. Every 12 hours, you get one mandate that you can exchange to a daily quest. They are doable as group but the problem is that you don't always get the same "daily quests pool" for each characters.
I did a little mining too. Multiboxing is not overkill for mining but it's definitivly better than wow for exemple.
Each char as his own nodes, sometimes when you target a node and interact, two, three or all (if you are lucky) of your char get the same. If not, they own node is close by.
I don't know yet if there is any profit to make from mining/herborist but if so, that's interesting.