I've often wondered why more games don't use an Eve Online style of progression. While I don't think time alone is necessarily the best method of doing this, I do think having a much longer ladder to climb would be more effective. In short, keep leveling small, but make the level cap extraordinarily high. So high in fact, that it may not even be given a number to quantify it, instead simply boosting various stats or skills. That way players do not lose the feeling of making a difference on their own character, while also not feeling pushed to reach the "optimal" point, as in a maximum level.
Of course, I also feel that a general stats platform of progress, while fun in short bites (like a single player game), don't really fit anyway. What about a system where character progression didn't result in your character simply being more beefy, or faster, or more powerful? Turn it into a more skill-based system. The point of this is to create situations where the "novice mobs" at the beginning of the game are no less deadly than they were when a player first started. They could still kill you in x amount of hits. Instead, your characters newfound skills and perhaps equipment provide a much larger array of tools in order to handle the mobs, allowing them to be dispatched with less risk or time invested.
After all, I liken it to an average soldier. Most soldiers can be equal in strength, stamina and speed with just simple (read: not easy, just simple) training. Instead, it's the skill of individual soldiers that place them in more advanced categories. One might be suited to sniping, with impeccable accuracy, while another is excellent at stealth.