1. Take away the uniqueness of Dark Souls
This one is pretty straigh forward. Because of the new arena, invasion frequency will be reduced drastically. And, in my personal opinion, invasions are the most unique and cool part of the whole Soul series in general. Sure, the difficulty is also very unique, and the insertion of an arena barely affects this. But once you get through your first one or two playthroughs, the difficulty becomes a non-factor. So really, you (could) be taking away one of the coolest parts
of the game.
2. Covenants become much less of a factor
Almost all covenants are PvP and invasion based, but, once you add an arena into the mix, why would anyone want to invade? Because of this, covenants could very well be almost completly removed from the equation of Dark Souls. The only reason people would join covenants now would be to get their items, and once they did, they wouldn't care about them anymore. And remember, this is speculation, not fact.
3. Ganker ratio increases. Alot
So, for every area, there are two seperate ganker to honerable ratios. One for invaders, and one for hosts. A ganker to honerable ratio is: for however many gankers you see, you will see an honerable player. So, for Burg, let's say, as an invader you will probably see about one ganker for every two honerables, so the ratio is 1:2. Make sense? But, now that an arena is made, the ganker/honerable ratio will skyrocket, and not in a good way. Why? Well, assuming that the arena will be a whole seperate mode, than you will probably get no souls for killing people. And, even if you do, phantoms will definatly not be allowed. Obviously, that will be a huge turn off for gankers, but a huge turn on for honerable players. Therfore, players who want a fair fight will congregate to the arena, and will leave all other in game areas. The result? No more honerable fighters, anywhere. Thus, the ratio will Explode. You will be seeing ratios of 30:1, 50:1, and even 100:1. That is 100 GANKERS FOR ONE HONERABLE. Insanity. I just hope I'm wrong about all of this.
Afternote: I will say once again, that this is all opinion. I am not trying to start a fight or anything like that. Thank you for your time.