Extremely slow and stupid hack-at-each-other-until-one-dies combat (which is nigh impossible to boot)
It's not an epic action-RPG, you're not supposed to blaze through it killing hundreds of mooks and monsters like a demi-god. It's supposed to be hard, and it's supposed to make you think twice before you choose to take the violent route (especially if the odds are against you). Your character isn't any Chosen One, and you have to put some thought into your stat placement and combat tactics if you're going to play a combat build. What were you trying to play as, and where did you put your points?
strange dialogues (okay, so I came in a tavern to see a trader sell something and then came back... why?)
Think of it as narrative, not just as dialogue. I'm also not sure what you didn't understand about that situation, it was very clear to me - Merchant X gets killed in the inn, the Innkeeper decides to sell of X's merchandise as he no longer need it, you go off to Fence and sell it, then head back to the inn.
extreme railroading (why can't I lie about anything?!)
The devs chose to include as little (in their mind pointless) filler dialogue as possible as they thought having three ways to say the same thing was a waste of both their resources and player time. It's been one of the things the closed beta testers have criticised the most, and I partly agree, but to what extent remains to be seen as I haven't had time to play much yet.
Also, you can lie whenever it's reasonable to do so, if you have the right skills for it. Don't expect to be a silver tongued magnificent bastard if you've but all your points towards combat.
remind me again, why exactly there's such a hype for this piece of shit?
Because not all people like the same things, and this was never marketed to a casual crowd.
After starting and restarting as the mercenary 10ish times to kill the assassin, its pretty much IMPOSSIBLE to do so. Even when I take out all my charisma and intelligence to buff up accuracy JUST FOR THAT FIGHT it's still impossible. Did they make that fight impossible on purpose?
It's not impossible, I managed to do it myself on my third-ish try, and that with a character that sacrificed both endurance and strength for perception and intelligence (combat skills was Dodge and Swords, but I also wanted a Streetwise touch to my mercenary). But then again, I played and finished the combat demo several times, so I have some prior experience.
There is always the option to stand down, though. There's not much point to fighting him either, you already failed to protect the merchant and you'll just end up hurt when you might need your health later, and while I haven't gotten very far yet making enemies with the Boatmen might have unfortunate consequences later.