An important thing is how a solution psychological affects the player.
Traditional leveling has proved its value here. Many people feel a rush and accomplishment when getting a new level, especially if the reward balances time spent. Parts of the game that adds to story, but not advance experience, may seem dull in comparison.
On the other hand, In Guild Wars I felt the same rush after spending a couple of hours in a monster infested jungle to finally defeat the tough boss that would give me an important skill. But does other feel that way as well?
For a gaming company this must be a very hard decision to make. To stick to a proven concept that 90% of the players already like or try something new that maybe less than 5% of the base would prefer.
