I know what you want, its tiered content. That does have some strengths, it gives and great sense of journey and accomplishment. Earning your way into Molten Core or Onyxia's lair was a feat to be proud of, let alone clearing them. But their is a huge problem with these as well. Tiered content has lots of bottle necks and creates huge hurdles for new players. Nothing makes a girl/bot friend love a game like "I'm sorry, you can't play with me until you finish 2 months of raiding after you hit max level." Bottle necks make it hard to progress to the next tier. You can't get a raid to help clear UBRS to key you to BWL because you are behind the curve? Too bad, try again later.
Also, you need "resets" tiered content due if you get too many tiers the hurdle becomes way too high for new players. Well that leads to another problem, such a small percentage will reach the highest tiers. As Zumi pointed out, less than 1% of vanilla WoW player reached Naxx. For someone trying and working hard to reach Naxx and then a "reset", ie expansion, comes out, that really sucks.
I will say, I did like vanilla WoW. I played it a lot and raided a lot. If FF14 had a similar path, I would be fine with that. I do accept they are walking a tight rope and not everyone is going to be happy with whatever decision they make.