There should be bosses/content in the game that are difficult. If every player can kill every boss, then what is there to separate a good player from an average one? MMORPG's are built on community and competition.
If you are just an average player, then like Mongo said you have a challenge to tackle and keep you busy for awhile, which means you don't run out of things to do as quickly. The satisfaction of killing a boss that killed you many times is tenfold compared to a new boss that you killed first try because it was too easy. I state that from personal experience of killing extremely difficult bosses from older games vs killing the watered down loot pinata's of instant gratification from more recent games.
You don't HAVE to kill every boss without wipes, this is what I think most people didn't understand when we were trying to figure out zork first couple times, people cared more about their repairs than trying to overcome a challenge. Guess why people used to play old MMORPG's for years yet basically all MMO's made in the last 4 years are beaten and discarded within the first couple of months? Because they were more difficult, the content kept you busier for longer.
However I don't think all world bosses should be difficult, but 1 or 2 of them should require a skilled group or co-ordination though and drop better stuff to compensate for increased challenge. If your stance on the game is that everyone should be able to beat every piece of content (communism gaming), then you are likely in for a disappointment when lvl 50 & ascended dungeons are released.
Edited by Malthrus, 05 March 2014 - 18:44.