Emphasizing my point yesterday, I will compare three trailers.
Original World of Warcraft
Original Age of Conan
Original Warhammer Online
Now, every one of them is fun to watch.
But the AoC and WAR trailer concentrated on something that you couldn't even do in the game.
Put yourself into the mind of a new MMO player. Not experienced like those who read this blog, but new to the scene. He will join AoC to fight like they did in the trailer or he will try to become Conan and strategize. Or he will join WAR to fight like they did in the trailer or be 'catapulted' like the orc. He cannot do any of this.
Look at the WoW trailer. Many races are introduced (not only barbarians), they do something you can actually do in the game: Looking impressive ;)
I'm not saying the WoW trailer is perfect in this regard. What I am saying is that the WoW trailer doesn't put the player in a mindset that is detrimental to his fun in the game. But this isn't just about expectations. For example, exploration was fun in classic WoW. Even if it is hard to imagine this today.
Have a look at this WoW trailer.
Honor, Mystery, Danger.
Apart from the 'danger' it is not bad. It makes you want to play for honor and mystery which is exactly what you could do in Classic World of Warcraft.
Now, remember the many WAR pre-release interviews by the developers. What they promised: Six cities, meaningful RvR. Remember what AoC promised: Sieges, player-built castles.
Remember the promised players housing which haunted Blizzard for years!
A trailer, just like all the pre-release hype and the game itself, need to guide the player to the fun parts of the game: Its strengths! Only WoW did it a bit. The rest failed miserably. It wanted you to do stuff that you cannot do in the game or it guided you to the un-fun parts of the game, like max-level of early AoC/WAR.
World of Warcraft failed later. For example, when they (significantly) reduced the exp you need for a level, they basically sent a message to the player base that said: Leveling is not fun, go for max level. What a disastrous advice - especially for any new player!
Developers must resist the temptation to put the assumed wishes of players into a trailer/ad! They need to guide them to the fun parts of the game, not the un-fun and unfinished ones.
Trailer are only a small part of this point and the whole point (1) is not the only reason for WoW's success. Far from it. It is one of many and a point where the competition failed.