One of the great difficulties of an open world game is that you can't really predict how the player is going to play it. Some play styles require the player to level up a little faster, other player styles might be more enjoyable with a very slow leveling speed. Personally, I don't like leveling fast. Once your character becomes really powerful it becomes a little boring for me. I like experiencing a more difficult game and a longer start up. The character struggling to survive rather than destroying everything. Other players play in the exact opposite way, wanting to blaze through quests, using a lot of fast travel, it can get to the point where your character isn't strong enough for the next quest - so faster leveling is great for them.

For me, that's why mods exist. They can't design a system that is perfect for every player, but we have the tools to customize those things.

I think a lot of how fast it 'feels' will also depend heavily on how the NPC/monster scaling works as well. In Fallout 3, I never felt like I was 'missing' anything when I leveled up because the enemies I would be meeting would be much the same, just a little tougher. Which is good. In Oblivion, I absolutely felt like I was going to miss half the game when I leveled up. Ding! Level 8 before I even had a chance to return the 'talisman' to the monks... I guess I won't ever see another cave full of goblins again...

yes, but I was satisfied with Oblivion's leveling. I just can't wait to see how the faser leveling works! its going to be different, but i bet it will be awesome.

Faster is a little better than slower int the beginning of the game. It should definitely start to slow and become progressively slower/more difficult after a handful of levels.