The second lesson we need to learn is that there's absolutely no shame in letting players see the entire game that they pay for. This isn't just endemic to MMOGs, either - John Scott Tynes discussed this last month, wondering why we so readily accept leaving games unfinished due to unreasonable difficulty. The competitive open-world aspect of the first EverQuest meant that unless you had literally hours every day to dedicate to the game, you were never going to fight the big bosses. (Again, see poopsocking.)
The advent of the instance changed this - now, if you had a chunk of time to play the game, you could go adventuring and get the exact same dungeon experience as the hardcore folk, but at your own pace. You never had to worry about getting camped or beaten to the kill. Of course, there were other obstacles to consider, now - let's take WoW, for example: In the original World of Warcraft, if you didn't have 39 other friends who could show up every Sunday night, you were always going to be a second-class citizen (unless you could do the arguably-more-soul-killing PvP grind, which was a chore in itself). The hardest part of those beginning raids had nothing to do with the bosses or their underlings; the hardest part was getting the 40 people together in the first place.
Of course, since then, WoW has been made considerably more casual-friendly. 40-man raids gave way to 25-mans in Burning Crusade, and were given a 10-man option in Wrath of the Lich King - with the overall difficulty and minimum requirements being drastically lowered. In Cataclysm, all of the horrendously complex stats are being simplified to make the game more streamlined and less of a spreadsheet. All of these changes have been met with cries that Blizzard is catering to casuals by "dumbing down" the game.
As damn well they should be. There are eleven million people playing this one game, and how many of them ever cleared the original Naxxramas? How many defeated Kil'Jaeden in Sunwell Plateau? If Blizzard, Turbine, BioWare, or NCSoft dump vast amounts of resources into creating this high-level content, those resources are ultimately outright wasted if only a tiny fraction of the playerbase ever gets to see them. If I'm venturing into the Mines of Moria in LotRO, I want to fight a freakin' Balrog - and I don't want to have that dependant on finding fifty-nine other people. These "dumbed down" changes make it so that more people can see the content that the developers bust their asses on, and make it so that more people can see complete storylines from start to finish. What's wrong with that?
Of course, this doesn't mean spitting in the face of the hardcore crowd, either. There do need to be challenges available to keep hardcore players interested and to make them feel rewarded - but it should just a different, harder version of the content everybody else gets to see. Give hardcore players "ultra hard" modes - the Ninja Gaiden on Very Hard to the casuals' Bayonetta on Automatic - that reward the victors with superior loot, and special credit for their virtual dick-waving contest. If they want a challenge, give them a challenge, but in this day and age, it just doesn't make sense to waste development resources on a tiny fraction of the playerbase, and then bar everyone else from seeing that exact same content.
And if they don't like it, well, then they can just go play EVE Online.