No leveling and here's my 2 cents why:
A lot of time needs to be invested into creating leveling content. Quests, NPC(s), Hubs, dealing with bugs,etc. It goes on and on. I'd rather that development time spent on the end game content right out of the gate. Even if creating leveling content takes 1 minute away from end game sustainability and design, it's 1 minute that can make the difference between an good event and a great event.
Also leveling content is a finite commodity. Once you do it once, you're done. The value it adds for replay-ability only caters to alt-aholics at that point unless you start crafting even more leveling content than is needed. As example, let's say I need 3 quests to level up, but in order to increase replay-ability of the leveling portion of the game, I have 5 quests I can pick up. The efficient player will do 3 and skip 2. The completionist will do all 5. The alt-ahoic will make a second character and mix in the other 2 quests. In the end, all 3 groups of players will complain about something because that's what we do as a player base: complain. At this point even more development time wil need to be spent fine tuning leveling content pull away from other projects.
Leveling in MMO's is nothing more to a grind in my personal opinion (can't speak for anyone else but myself here). Skip it and focus efforts in content that matters.