Vertical Progression. The idea that there is a max level but content behind that also.
People who dont understand this are people asking for a free to play game. You want more content? Or do you like working for days and days to get to max level and be told "ok now that character is done go make another and start at lvl 1." Plain and simple this is not a game where small bonuses can be gained through experience that will give you an edge in the same setting you are always in. If you want free to play go play a MOBA because you WILL NOT be happy with a game that does not have the funding to support a team who works on improvements and content so you don't get bored. This is the reason making an MMO is scary right now, when you lose your subs you lose everything.
Scaling. Games that scale up huge amounts through level cant stop that scaling for max level.
Hopefully Zenimax will not scale the game to a point where nothing feels epic. If they are going to make an MMO I hope they do it right as far as progression. AoC and SWTOR both failed at scaling and end game. I am not saying they need teirs and teirs worth of stuff ready but if you can down all content 1-2 weeks into being max level it will drop your subs through players who are not just here for dominating PvP and kill the game.
PvP. A huge part of the community wants to dominate in PvP.
If this game is centered around PvP you will fail. It is that simple. Balance around PvP and you have another game in the boat of, "loosing subs fast and now we have 5 people on a server at once which makes those 5 leave and so on so on." Psychologically you need to win 60% of encounters to feel "balanced." With 1 person winning 60% of the time that means another person is only winning 40% of the time leaving you with a 20% gap that you CANNOT close. 60+60 will never equal 100 and that is what leads to games loosing such huge amounts of the community at a time. You loose half your Subs cause PvP is pointless and this other new game is going to be more balanced (not really) so that half leaves bringing up a new whole which will quickly be split into half also. PvP based MMOs just don't succeed.
PvE. Your strong subs that stick are the theorycrafters who seek difficult endgame content.
What I mean when I say that is the exact reason I left WoW and SWTOR. I like HARD content that I have to coordinate with a team (I'm sorry BioWare but 8 man ops is a joke.) People who will stay in the game are the people who do grueling quest lines so that they can qualify to enter content. Content available to everyone is content that is able to be killed by anyone and everyone. Please Please Please I beg you to consider real progression through endgame content instead of "there is a raid in this zone and I want that piece of armor and there is a raid in that zone that will give me that weapon. Now that I am max level I can go and get them yeah!" Don't jump on the bandwagon of making content have 5 levels of difficulty and counting that as a challenge. I just killed this guy in blues and it took 5 minutes so I will get purples and kill him again in 5 minutes so I can get even cooler purples.
Epic Feeling. The Ultimate success of an MMO is a feeling of Epicness.
Really this is what the whole post leads up to. How can you feel epic when there is no challenge or you feel there is no balance? "OK so I want to be epic I must choose class X for PvP cause right now nothing counters them. But oh wait everyone is now that class and again it is not epic." Incoming nerfs leading to another FoTM class and play style that you don't really like but you get stomped without it. How do you feel epic when you know you can just see the inside of this place with a casual group who also wants to see it? There is nothing epic about "ding max level ok lets go get the coolest best looking armor! Time to ask around and see who wants to do a 5 man group with me to kill this guy that I will need to kill again and again with bigger groups or flipping a switch to increase his damage and health by 20%." There NEEDS to be progression and not just increased numbers for the same work.
Bottom Line. Dont Rush and take your time.
Choose what you want this game to be. If my insight is way off for your own sake as a company tell me to get lost. You examine what you want to be known for. You decide if you want to make the best PvP MMO out there or if you want to make what would ultimately be THE ONLY MMO that gives quality PvE content. You choose and you do it! Don't let the idea of money rush you into releasing an incomplete game. You make the best you can and hold the game until it is there. I want to see it in 2013 as bad as anyone else but for the love of all that is holy PLEASE DON'T release a game to be torn up and spit out by your community in the key weeks and months after release. First 2 months will make or break this game. MMOs are relentless and it is playing with fire. Please don't get burnt.
Any feedback will be loved and Please feel free to add your bullet points to the list.
TL:DR- Look at bold points everything else is explanation.