It mainly the graphic design and artistic look that Del Toro brings if u ask me, all the creatures, buildings, costume styles, weapons etc...they all have a certain style that is uniquely Toro, right down to pattern designs on doors, weapon handles and belt buckles. A change to this could ruin Hellboy and I for one am against it.
Must admit I am intrigued to see Hellboy in a Tim Burton style world, visually it could be stunning.