The ability to feel powerful is much harder to come by in an MMO than a SP RPG. In Skyrim, for example, I spent many hours perfecting my character to make him a complete boss. I'm not talking about exploits (like one hitting legendary dragons), just careful natural leveling. By the end of Skyrim, I could take on legendary dragons and giants at the same time. I could slaughter the entire guard of a town unaided.
But in an MMO, there are always other players. These other players will be on an equal footing with me. This makes it much harder for me to feel special/powerful. Obviously you need to preserve some equality for PvP and general fairness. But I still want to feel like more than just one among thousands.
The way you do this is through PvE. Starting zones must be difficult (think Morrowind here. Who remembers this cave? http://uesp.net/wiki/Morrowind:Addamasartus). As a low level character, I shouldn't be able to survive outside of the starting zone. Monsters in the immediate area should easily be able to kill me. That way, when I am strong enough to travel into other zones, I will feel as if my leveling has made me meaningfully stronger. Finally, when I reach level 50, all normal PvE combat should be easy. I should be so powerful at this point that now monsters found while traveling could even hope to stand up to me. Only my fellow heroes in PvP and the special mobs in elite dungeons should even hope to be able to stand up to me.