It's going to be like Pokemon. Throughout the game world they are going to have dynamically spawned exploration rewards/boss/mob rewards that will reward you with a different class subset (regardless of weapon) that will open up a new series of skills that are gained through using the prior skill. Based on a drop percentage. The weapon skill set is separate from the class. You will be able to use any weapon you want. So it makes the skill sets endless. Basically what they did here was multiply the GW2 class system by two.
The destructible terrain was done once before, and is now scrapped. I'll give you a hint, it starts with a T and ends with an N. The problem here is setting up the stage effects to getting to other content aside from just looking around random terrain, which is boring as shit. And you cannot path the NPC's properly in these locations unless the spawn is done dynamically, which they are still trying to figure out here, but it isn't doing the game any favors. It creates an empty "fake" feeling of, "Hey here is Lavastorm... and... now... it's completely different than the last Lavastorm I was just in because some guy did 56 whirlwinds and knocked down everything - which then randomly regenerated in a different form 5 minutes later. That's great in a sandbox game. It's not good in an IP that traditionally been about living in a persistent world. And most memories of gamers are made by remembering actual places. That's going to be pretty hard to do when everything will be dynmically designed on the fly, along with mobs, and really, no reason why the hell they are there. Ever.
There isn't much new here besides a terrain builder, an instance to house that in away from the "static" game content, and they haven't announced the fun little details about the cash shop pertaining to accessing that player made content yet either.