Meh. There are certain 'milestones' in mmo's and usually the 'halfway' point is one of them, somehow being level 15 just doesn't seem so awe inspiring. Part of the whole system of why mmo's even work is that it takes you at least some time to reach the max level. "End game" content shouldn't be the only important content.
Since I've been using Champions as my prime example, I'll continue with that. At level 30 in champions you are still a bit off from being max level (which I think is either 40 or 50, can't remember) but you still have a lot of powers and around this level you start doing some epic quests where you fight through a story based level and then fight a miniboss and ultimately fight a super villain (boss) before the quest is finished. It's definitely a milestone and something you can look back on and go "Man that was really awesome." Of course towards the endgame there are more quests like these that pit you up with the world's super villains, but those mid-range battles are something you never quite forget.
Even in non-Super Hero mmo's, leveling is usually a process and there's a point where you feel like you've accomplished something by getting that level even when you know you still have more to do. For example, getting level 50 and being able to start your artifact quests in FFXI and then by 60 finishing them all and wearing your new armor even though at the time, you still knew you had to get to 75. In WoW, getting to 60 and getting my first flying mount felt like an achievement, finally gaining far more freedom in travel even though there are other achievements to get as well and 85 is the max level.
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