The Witcher 3 launched to much acclaim from both critics and players alike. The game struck at the heart of what an RPG should be. It had a myriad of weapons and armour to use, an epic story, decisions with real consequences and a massive open world. The world is huge and dense, with many different stories to experience in every village. Despite this, I only managed to experience a small piece of this world, The leveling system does not allow much wriggle room in its recomendation for the main quest line.
After wandering Velen, one of the first areas available to explore, for a short time, I found myself overleveled for the rest of the game. This took away from the overall experiece as I no longer wanted to explore. I felt like I had to progress in the main story, as being overleveled made it far too easy. For me, the ideal way of playing would be to wander aimlessly around the world between main story missions, but because of the way the leveling was set up, this was discouraged. I ended up completing the game, and losing the motivation to explore the rest of the world.
This problem with leveling can be seen in other popular RPGs like Dragon Age Inquisition. Dragon Age Inquisition also expected the player to speed through the main story, and explore the rest of the game after completion. Personally I enjoy thoroughly exploring a game before finishing the main story.
Particularly with The Witcher and DA:I the main story builds up to a final battle, and after this it seems pointless doing all of the side quests and collecting all of the loot that is left. DA:I caps its final boss fight at level 19, meaning that it is very easy to overlevel if you find yourself getting caught up in the many side quests that the game offers. The final fight then very easily changes from an epic finale, to a disappointing beat down.
RPGs should take a long time to complete and encourage the player to explore the world. Instead with the leveling systems implemented in both The Witcher and Dragon Age, the player is encouraged to run through the main story before exploring everything else the game has to offer.
Leveling as a mechanic can give the player a sense of progression, but when it is possible to over/under level your character it can be very immersion breaking. The Dark Souls games show off a leveling system that does not restrict the player in this way. In these games it is possible to play through the entire game at level 1 (although it is quite difficult to do so), and levels act as a means to an end for the player.
The inherent difficulty of the game means that players rely on their own skill, rather than the characters level. Leveling certain stats allows the player to use different equipment and make use of this equipment more effectively. This progression is slow and so players do not need to worry about becoming overpowered. This allows the player to thoroughly explore the world, without fear of overleveling.