Barriers to entry is originally an economic concept, describing how competition can be kept out of a market if for example it takes a huge investment to enter that market. But for a MMORPG player the term barriers to entry describes the difficulty of starting a new game. The recent announcement of Age of Conan being delayed for half a year specifically cited those entry barriers as something they still need to work on. Age of Conan has developed a brand new way of doing MMORPG combat, but apparently it isn't very intuitive, and there wasn't a good enough tutorial to get people into the swing of things.
My personal barrier of entry experience of the last week is Everquest 2. After not playing it for nearly 3 years I have forgotten a lot about how that game is played, and could have used a good tutorial and introduction into how things work. Unfortunately I started in the Nursery, the newbie zone introduced in the Echoes of Faydwer. While EQ2 has excellent tutorials in the original island newbie zone, the devs took some shortcuts and left out many of these introductory parts in the Nursery. This weekend I started crafting. There is a series of crafting quests to get you into things in Kelethin, but these quests contain zero information about how crafting works. First quest is "bring me a fried frog leg", telling you that you fry a frog leg on a stove. So I go to the stove, find the recipe, find the guy who is selling the wood needed to make a cooking fire in the stove. But I can't find a frog leg to fry on any of the vendors. And I didn't see any frogs out in the forest from which I could have gotten one. Finally I ask in chat and it turns out that I need to find a "school of fish" in the ocean bordering the Nursery, and click on it to fish, which after some tries gets me some frog legs. Sorry, but fishing frog legs from the ocean with the fishing skill isn't really intuitive. Some explanation would have been great. I should maybe make a new character just to do the island newbie zone once and learn how everything works. Well, I got my crafting up to level 10 by trial and error. After being hit with accident events for several levels, I finally found the hidden crafting tab in the knowledge book where the icons are that you need to counter those accidents. But some explanation of all that would really have lowered my barrier to entry into EQ2 crafting.
In my opinion the two MMORPGs with the lowest barriers to entry are World of Warcraft and Lord of the Rings Online. Both have excellent newbie zones explaining you just about everything you need to know to play the game. Thus if somebody would ask me for advice on what he should buy as his first MMORPG, I would recommend either of these (depending on how much of a Tolkien fan that person is).
Many other games get away with not having such a good introduction, but doing most things very similar than previous games. Every MMORPG which has a map uses the "M" key to open it. The keys for other activities, like opening your bags or quest log, have not been the same in all games until 2004. But since then, World of Warcraft has become the gold standard, and many games just copied the keyboard layout of WoW. Of course this strategy only works if your new players actually played WoW or a similar game previously. A big part of the success of World of Warcraft is that it has such low barriers to entry, and therefore succeeded in bringing many first-time players into the genre.
Being accessible to newbies is very important. You can't just assume that every single one of your players has played WoW or a similar game before. World of Warcraft has 9 million subscribers, but that is far from the total number of people playing video games. The Playstation 2, for example, sold 120 million units worldwide. A new player has to thoroughly enjoy his first hours in the game to get him hooked. If he spends that time wandering around in a bewildered state, you'll have problems keeping him as a customer. You really don't want barriers to entry keeping players out of your game.