I'm actually inclined to agree with you about 8-way movement not being a problem (the camera is controllable), but a lot of people voice that as a concern. There's some tricky almost-platforming segments in Dark Souls, so I could see why they voice that concern.
As for button mapping, Dark Souls (and Demon's Souls) utilizes every button the gamepad provides excluding left-stick-click and every function matters. And whoever would port the game would have to keep in mind that not everyone has mouse 3, 4 or even a mouse wheel sometimes! That being said, here's how I'd map the controls (from 360 to PC) off the top of my head:
Left Analog Stick > WASD (move)
Right Analog Stick > Mouse (camera)
Right Click > Tab (lock-on)
RB > LMB (right weapon normal attack)
RT > RMB (right weapon heavy attack)
LB > Shift (left weapon normal atk/guard)
LT > Ctrl (parry, left weapon heavy)
A > E (interact)
B > Space (roll/backstep/sprint)
X > F (use item)
Y > Q (dual wield right-hand weapon)
Back > G (gestures)
Start > ESC (in-game menu)
D-Pad Right > 1 (swap right weapon)
D-Pad Left > 2 (swap left weapon)
D-Pad Down > 3 (swap item)
D-Pad Up > 4 (swap spell)
Note that the in-game menu for Dark Souls would present some complications. It doesn't pause the game (think MMO) and you can move around and rotate the camera while in the menu. That creates for the need to keep both WASD and mouse to navigate menu
If that setup looks clumsy, it's because Dark Souls is a game that really fully realizes gamepad controls, and the conversion is awkward. Hopefully not too awkward--I'd love to see this game ported--but damn, it's going to be a hard one to do right.