Greetings!
This is written in the chance, slight as it may be, the developers will get to have a look at it.
There are some key points i would wish to write down, as the genre's history has proven that times and times again, somehow, developers ended up ignoring them.

1. Audience: MMOs do well when they have as a target audience the dedicated (not the casual) MMO player. No, this is not so self evident as it may seem: The dedicated MMO player is one who has both the time and the will to invest multiple hours a day, every day, every week, in order to play 'a' MMO to its fullest. He is looking at a minimum of 3,4 hours a day of a log in, going all the way up to even 9 and 10, though i could push it even further. This is the kind of person you are aiming at. Why? Because the casual, precisely due to his nature, has neither the investment, nor the desire to ever go ahead with it. As such, he has little to lose, and even less to feel sorry about. No strong connection. He is the one that will leave to check out something else, perhaps return, perhaps not. He will not be the base if you will of your income. Be it monthly or store-related.

With the above in mind hopefully, a number of things come up:

2. Visuals: Be they gloomy or frolicky, bright or dark, cartoon-based or real-life, they must not be tiring to the eye. Your target audience has a 3-5 hour stretch ahead of it, and as such spell effects, weather effects, weapon effects and so on must be realised with measure. A balance between the aesthetically pleasing and that which does not tire the eye. Example being a very, very recent highly anticipated sequel of a certain title. Try doing 3 or 4 "sieges" straight, your age being over thirty (your average dedicated MMO player is around his thirties, plenty of research out there proving that). You cannot, for your eyes are so tired, you simply need a break. Until tomorrow. Alienation.

3. Sounds: Once again, the same principle. Especially with looping. Soundscore and effects need to be such as one can find it possible to listen to for extended periods of time. i.e. a horrid scream of a female villain that happens to be the end boss can be quite thrilling the first time, but soon becomes tiring, if not the source of a good joke or two regarding her sixual deprivations whilst chained by the nasty developers in some rot-infested dungeon. A good laugh afterwards, people are prone to turn their sound off. That's alienation right there again. Keep anything excessive out of the looping, and/or make it occur but once per encounter, session, or what not.

4. UI UI UI: You won't get everyone to love it, no matter what you do. But you must make everyone be able to acclimatise with it. As such, it just -has- to be -both- scalable, and movable. In all of its aspects. If you cannot grasp why, or think you know better, look below.

5. Hotkeying: Everything has to be able to be reconfigured, reassigned to a key of the player's choosing. Including the tilde, the mouse buttons, everything. Your dedicated MMO player, which will show up expecting an MMO and as such will be ready to make relevant comparisons with older ones, has by now his formed habbits, his methods. He has been playing for a very long time titles such as your own. Finding the freedom to assign his buttons as he had in his previous game will make him more happy than you can know. Remember, this is about a stretch of hours. To him, this will matter. Ease of familiarity.

6. Combat:

a) Whichever way you go with it, it again has to be as less tiring as humanly possible. Physically speaking once again. Recent example that comes to mind is a new MMO that did not go very well, despite being made by a famous single-player games company, and having borrowed a world famous IP. Combat there amounted to a keypunch fest, where the strongest and most flexible fingers won the day, rather than well..anyhting else really. Multiple mobs constituting one, single fight, with the result of the player needing to press the same buttons multiple times just to end that one encounter (that was their idea of epic, splitting one mob into 4). Now multiply those already multiple times with the amount of times this would have had to happen if your dedicated MMO player was at the chair..impossible for a long stretch..failure right there, just due to that.

You seem to be taking the smart way, with a minimal number of skills ever active. Do not change that. Firstly, it adhers to the above, secondly, it promotes strategising.

7. Activities Outside combat: You need them. The more you offer, the better the game sticks. Look at EQ2, deeply. And you will understand how a game that is so bug-ridden, poorly coded and haphazardly expanded each and every time can -still- manage to retain financial benefit for its publisher. Activities. More than one could have time for. The less you put in at launch, the worse your future. Simple as that. Age factors alone indicate a need for some form of an intellectual process taking place as well, if not social.. this has to have tools that will help it come out, so bonds and communities can be formed. No, people forming guilds are not communities. Communities are what you see inbetween guilds

(housing should make its appearance here too, but as you stated it would be out the question..for your information, people at forums laughed when you said you cannot put housing into the game..you can..you just do not care to..)

8. Balance: Speaks on its own, but this is more on the how you'd go at it..simply? Take advantage of your potential customers. Your first beta, have it show the PvP aspect -not- the PvE..that will be the pool of all your observations, and the foundation of each and every problem you will encounter in the future. I am really unaware as to the why everybody does Betas with PvE content alone, at least for a start, but it should be the other way around, if you persist in having both a PvE and a PvP element in your title. Balance. And it shows best in PvP. Each and every weakness or problem, it will spring through PvP activities. Save your PR people a lot of hassle and woe by doing your Beta the right way. It will pay off. Just have a look at the behemoth's forums (starts with a W) everytime a patch comes up, and a class is affected. You will get my point.

And lastly, do not make the mistake of thinking you know better. If a majority of your forum regulars vehemently suggest, or critisise something, listen to them. They do know better. Chances are they have played MMOs for far, far longer than you.. empirically speaking alone, they do indeed know better and as such should be heeded.

Assuming anyone belonging in the developing dpt ever got to read this far, my sincere thanks..
Wish you well