I am sure that there are many views on the questions you've just asked. But at least I'll give you my answers and I hope there will be more people whp follow suit, so you can really make up your mind.
1) Difficulty is not that high, I find them challenging at first, but very soon repetitive. I think this is also the case for the leveling system. Again a bit too easy, and perhaps too many quests.
2) Nope, it is really a great combat system. I still enjoy it very much and on top of that, playing different classes also gives a real new feel to it. In WoW for example it could 'feel' the same to play any caster. Here at Tera, this is not the case.
3) Balancing I am not too sure about. This due to the fact that I have not been playing PvP that much. Besides that, I like playing a Lancer, not the best class to play PvP with. The diversity is great! Again the combination of classes is really different to play, but also the race combination does give a feel that you are playing a different person.
4) Since we do not have Battlegrounds yet, there is no telling in how this will turn out for Tera.
5) Combat system is the best aspect. Also the grafics are amazing, but that should never be a factor. I like Minecraft as well, and the grafics there are nothing to write about.
6) I personally think the leveling is too bland. I do not feel gripped to the story, too many quests and I have lost track of my characters motivation. Now it is just a accept quest, kill target, get XP. Next to the leveling, the crafting is also quite a task. BUT I do think these things can and will be fixed in the future. It is a 'new' game, WoW was not build in a day either.
Hope it will help, but my biggest answer is yet coming. Just play it for a couple of months. You won't be dissappointed. But to be honest, next to the combat system, it is nothing different from WoW.