In my very personal opinion and experience , new games should avoid a few things .
1. Action Combat doesn't work in an MMO . This will again be tested when ESO and Wildstar launch . In my last thread I asked for examples of successful action combat MMO's and people gave me Darkfall and Tera . This alone would scare me if I was developing the game. I don't know why action combat doesn't work in an MMO. It may have to do with the inherent repetitive nature of an MMO. It's not like a single player game where you will go do clear a dungeon and move on to the next area and the next battle. In an MMO you will do the same zone 10's or maybe 100's of times. Just swinging a sword , or moving around a bit isn't enough to keep people engaged when doing the same encounter over and over again. Come to think about it , not only there is no example of a successful action combat MMO's but even the trully successfull action combat games failed incopareting an MMO element ( Diablo 3 comes to mind ).
2. Quests , Dungeons , Faction , Raids , Tokens are all time sinks that aim to keep a player subscribing to a game month after month . All of them have as an end result either getting a nice item and improving your character or killing a new mob most people haven't killed and feeling good about it. In the long run nothing else works. I have been in many games and I remember people always feeling expectant about new stuff like Vanguard's Diplomacy , WAR's Public Quests , Rift's ...Rifts , Aion's Flying Combat , SWTOR personal storylines etc. In the end I tried them , mostly liked them , and then went to start grinding items , faction etc. The game with good time sinks ( Rift , Vanguard ) I kept playing , the rest not so much .
3. Community and how to build it. I think at some point developers have to understand that they are not competing with single player games. They have to ask themselves how they lead their customers away from spending their times doing solo quests ,until they are bored and leave the game , to actually engage with each other and start building a community . I met more people willing to help me to get from Antonica to Commonlands in EQ2 at launch , than in 2 months playing SWTOR . I mean seriously the only thing you actually need help with were instances and a couple of group quests you didn't really need to do .
4. Endgame . No subscription based game will survive long with no endgame . Be it the huge pvp fleet battles of EVE , the end raid mobs of WOW and EQ etc . It's important not only to keep the playerbase occupied and involved . It's even more important because if there is no endgame , for whatever reason ( not ready , buggy , we don't need endgame , etc ) , there is gonna be EPIC mauling in the mmo forums . The amount of hate games like SWTOR , Tera or GW2 got in forums was staggering . After the honeymoon month passed they were thousands of people that made it their purpose in life to destroy the games . I am taking about pathological hate . I am talking about people logging throughout the day to check server status in order to prove the games was bleeding players.