I am extremely excited about getting into TESO. I have played most of the big MMO's and seen them come and go. I can only speculate as to why they failed but I don't have to speculate as to why I left them. So I just wanted to touch on a few pitfalls to avoid in the future.
1) Balancing rollercoasters - Rift had a huge balance problem. I remember standing on a bridge with a buffer friend of mine and blasting whole pvp armies away with just my pyromancer. While it was incredibly fun it began a roller coaster of balancing between the classes where every month one class became OP and another UP. There was no stability to your class, and eventually you got the feeling you had no control of your class and lived in constant fear of the game breaking Mega Nerf Bat waves.
Lesson - Make small changes first not large ones.
2) Free to Play - It only starts out all cute and cuddly. I have played Lotro for 5 years. Great Game, great RPG, great MMO. F2P opened the door for more players, and perhaps prolonged it's life span. It certainly made them a lot more profitable. It is still doing well but It lost touch with its original player base, it's slowly losing its soul. At some point that cute cuddly teddy bear gets it's financial paws wrapped around you and takes over.... Design starts to shift from the designers and moves toward the sales team. Rather than "what cool dungeon can we design" it becomes "Could we design a new horse and then sell that horse for more money". At some point it completely takes over and the sales team starts designing the game. The world that was originally made dies in a whimper and then all that is left is a Mall with a playground in the center.
Lesson - Designers, Programmers, you love the game you made. If you go F2P you will lose control of your IP…forever.
3) PvP Integrity - Honoring achievements is important. After playing lotro for 4 years I reached a pretty high and respected rank for my class. At which point the developers decided to inflate the ranking system, so that what took me 4 years is simply achievable in half a year. In addition, "multiboxing" legions (1 player controlling an army of characters simultaneously) and rampant "fightclubbing" (farming yourself for rank using two accounts) are left unchallenged by the administrators. With no integrity to an achievement or to gameplay it renders the entire endeavor worthless, and breaks the immersion. It makes the whole idea of that portion of gameplay a fraud and a sham.
Lesson - Hold to the intent of the game and protect every part of it from exploit. Each breach breaks the immersion. It causes a person to start thinking outside the game rather than in it. Instead of being immersed in a world, they become immersed in the reality of how broken and lawless it is. Protect the immersion.
4) Longevity - However long you think it will take a player to reach point X, he will legitimately reach it in half the time you think. When Rift came out they grossly underestimated how long it would take a player to reach the max pvp rank. To keep the players busy they kept breaking the ceiling, adding more ranks and absurd performance rewards (+X% pvp dmg, and -X% inc pvp dmg). Creating an ever increasing gap between the new player and the veteran. It quickly became untenable.
Lesson - Make your ceilings really high, then make them a little higher. Create an max allowable gap between your new pvper and your vet. Performance rewards are important, but don’t break the gap. Plan ahead, so that by the time you reach that gap it can be refreshed by a level cap increase expansion.
5) Buggy updates - Test, test, test, before release. Games are complex and we expect a certain amount of error, but it's important to keep a trustworthy reputation. I don't want to keep blasting the same games so I will speak anonymously for now. One game I played has a reputation for releasing content with bugs so obvious that it gives the impression that the content was either not even tested or they just don't care. What I mean here is obvious bugs like, the dungeon you just fought through doesn't give any loot, ever...!!!. (shocking as it may seem stupid bugs like that were expected in this games releases due to the reputation they built.)
Lesson - It's better to release late than ruin your reputation and the trust of you player base.
Those are some of the biggest pitfalls I have seen. Just a few things to keep in mind for the future. Looking forward to being a long time player and supporter.