A member of ArcheAge Source has posted a long overview of the game. Let's talk about levels.
Leveling up in Archeage is loaded with quests, there's no doubt about that. However, given that there is a lot to do in the game with, for example, trades, exploration and of course leveling up, it is difficult to focus only on quests. You will also have the opportunity to do many other things: cut trees, collect ore and refine it, find recipes for trades and construction, and sell them .... All this gives experience and allows you to 'forget about quests for a while, giving a nice reward on top of that.
If you don't want to do this, killing monsters is also rewarded: there are gang research quests that allow you to get bonus experience and some potions that make you stronger for a while (can only be used 'at a certain level and not resalable). While the quests are a lot of fun, the majority of them involve running around and killing monsters.
The story of ArcheAge, as well as the storyline of the quests, is based on a famous Korean novel by Jeon-min Hee, which means that while sometimes the quests are a bit repeatable, the story is great there is lots to read and explore with currently 2 books on ArcheAge.
The difficulty is rather low. The record for reaching level 50 is currently 12 hours and 34 minutes. However, it is more difficult to show all of your trees at level 50 for the simple reason that you have fewer quests available.
When you create a new character, you get help from the game with level 20 gear and adventurer potions which greatly improve efficiency. You don't have all of that at a high level and you have to work hard to get tiny amounts of experience. The real challenge therefore does not consist in raising your character level 50 but rather in climbing all of her trees.
