i played the demo, but besides leveling and skill tree, it didnt really feel like an rpg. is there at least an exploration aspect? are there any towns or secret areas? is it just a linear game? quests? is there anything else to do? can you choose to not follow the story? or am i to assume that this is an "action adventure game" like god of war, but with a leveling system and skill tree?

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It has a few hidden areas but its supposed to be an combination of an action game and a RPG. The RPG elements come in for leveling and the skill tree as well as the enormous amounts of loot that you get. The combat it more action/skill oriented than most traditional RPGs such as Diablo or the Final Fantasy series.
I hope that answered at least a few of your questions, but im not to sure about the the alternate quests.  

There is exploration in the hidden rooms/arenas (there are 4 in the demo, have you beaten the all?), as well as by activating and vising cyberspace wells.
There will be a central hub city Aesir, that you can/will visit between mission and likely talk to other characters, and purchase/fix/upgrade weapons and armor. It is not really known what in the way of side quests there are in the game, at least not in any info Ive read lately.

Story progression is probably going to be pretty linear, as SK has been pushing the story telling elements of this game trilogy for a while now. I fyou want to skip the story entirely, there is always online co-op, which cuts the cut scenes and is all gameplay.

yeah the rpg element were not really in the demo considering that you only got up to level 7 (or 8 if you maxed out your combo meter and got exp gear) but in the later levels skill choosing will get a bit tougher it seems.  Also it seemed pretty obvious which gear was better in the demo if you had any prior rpg experience, I'm Sure this won't be the case in the final product.

SK has hinted that the alignment you choose, Human or Cyber may have an effect on the story.  So not like Fallout with its hundreds of endings but you can possibly have a choice between two depending on which secondary skill tree you pick.