There are two ways I look at the differences between a static level cap, and an increasing level cap, the first is character progression as a tree vs. character progression as a road.

The second is content that's maintains or loses validity purely on a personal level, vs, content that loses validity with every new cap increase.

The character progression in FFXI, was a tree... everything you did branched out from previous achievements, from leveling multiple jobs, to experiencing the multiple storylines, getting various new gear sets, it all contributes to the character.

In WoW, character progression is a road... you walk down the road, you enjoy the view, and when you hit the end of the road... you're at the end of the road stuck with the scenery there until they increase the level cap and you progress down the road again.

In the second aspect, player A may have done and earned everything there is to do in Sky and have no reason or desire to play through sky again. but player B may have only done a few things in sky on one job, and in playing a different job the desire to return to sky still exists... Player C might be a brand new player and is on his first 75 character, the rewards and benefits of sky are still available to him. Player B and C might team up from mutual desire for rewards from sky... and maybe they can talk player A into joining them. They still have the same challenge they had when player A was doing sky, it hasn't changed.

For a game like WoW, how many people are doing the level 50 instances and raids now? most of that content is largely overlooked as people skip by it to gain access to the items and instances for the current cap.

It's hard to write about them objectively, but it really does come back to ffxi held my attention for 6 years with its static cap of 75, and I still had a laundry list of goals when the game no longer became fun for me, and WoW never held my attention for longer than a couple of weeks, and LOTRO peaked for me after a few months...

And I'm not attributing FFXI's ability to hold my attention purely to the fact that it had a static level cap. But I am trying to impress the point that with a static level cap of 75 for 6 years, I never had the feeling that the game had grown stale.