Con Certain mechanics leave new players open to unfair PvP invasions
There are a few ways you might find yourself invaded by another player even when you don't want to be. The method for inviting co-op players into your world also leaves you open to PvP invasions, which you can't turn off. The other one involves how you can only carry five healing items with you at the start; there's a way to increase this to ten, but by doing this, it becomes possible for PvP-minded players to enter your world to challenge you to battle whether you like it or not. There's also an area a couple of hours into the game that's notorious for high-level players seeking out oblivious newbies to tear them apart, so beware.
Con Unforgiving and discouraging learning curve
Even though Dark Souls is technically fair, it's still really hard to get into. From the start, there's a minimal tutorial where you read messages scattered around the area for tips on the controls, and then you're forced into an immediate challenge that you have to figure your way out of. The game is clever in the way it pushes you into seemingly impossible situations and makes you think on your feet, but gamers who are new to the genre will more than likely find a serious culture shock here. Because the learning curve is so incredibly steep, you may struggle for a while before things finally click.