I've been playing Blade and Soul over the last week. I purchased a weeks worth of Premium to avoid the queues. B&S has a few more interesting aspects.
Abilities are very "context-sensitive". On my Force Master, using a Fire spell (left mouse button) puts you in Fire mode, and a lot of your abilities change to a Fire version. Using a Frost spell (right mouse button) puts you in Frost mode, and several abilities change to a Frost version. It's fairly easy to change contexts, to alternate between Fire and Frost.
There are other contexts as well. The FM has a "grip" ability that pulls and holds the enemy. While the enemy is gripped, several new variants of abilities are unlocked.
This is then backed up by the talent tree system. Each ability (or more accurately each button), has it's own talent tree. The tree usually has 2-4 mutually exclusive branches that changes or enhances the ability in a specific fashion.
For example, that grip ability I mentioned can be changed into a snare which prevents people from moving. Another short-range ability can be changed into a variant that does less damage but has a longer range.
All in all, the ability structure of B&S is very interesting, and it's worth looking at to see how a large number of abilities and variations can be mapped onto a small number of keys.
The PvP devs of most MMOs have said that they do not balance around 1v1 PvP. B&S embraces 1v1, and has it being the central format of PvP. I've only tried one duel, and lost fairly quickly. But it was a pretty interesting experience.
Dueling has a long history in WoW. You can often find people dueling outside the gates of capital cities. After playing B&S, I am no longer certain that simply writing off 1v1 as unbalance-able was correct. Perhaps if 1v1 was balanced, that would simplify the balancing of larger groups, or classes in general.
Of course, B&S doesn't really have dedicated healers, which changes things significantly.
In most modern western MMOs, health is a per-encounter resource. You start the encounter at full-health, and it's fairly easy to get back up to full-health afterwards.
In B&S, health regenerates far slower, but you generally take less damage during a fight. Resources to recover health are moderately scarce as well. So you generally deal with multiple pulls on one health bar, and only rest when your health gets low.
Those are some of the mechanical aspects of Blade and Soul which I found interesting. The queues have died down, so it's a decent time to check it out. Really the only issue is that the spammers are out in full force, and have overrun all chat.