For this game to be truly epic...

The big three of any game for me are. 

Number 3: [Strategic/Tactical Depth] This increases a game's re-playability and fun factor overall for me.

Number 2: [Gameplay] Great gameplay in addition to reducing potential frustration just makes the game more inviting and fun. 

Number 1: [Plot] I'll put up with a lot of crap for a good plot unfortunately they are few and far between in most games although there are still a good number of games with good plots. 

For me the short answers are:

1. Killer AI Action
2. Wicked Customization Options
3. Extreme Immersion Factor

1. Number Uno for me is fun...thats it..just plain fun.  Make it fun like MoM was and the early Civs and you cant go wrong with it.

2.  Make the tech(research) trees massive and different.  Make each element have different ways that it can be used then other paths.

3.  I also like the Immersion Factor...make it real to me and that I have a vested interest in my game

Hmm...thought awhile on this one.

1. Everytime I play this game I want it to feel different and engaging. Not the same boring grind of base building and micro managing of units over and over again.  I don't know this can sometimes be the feeling of other RTS/Turnbased games out there..BUT this is SD we're talking about!!! So I'm sure this won't be to big of an issue

2. I like the massive tech trees and all but I wouldalso  like some kind of spell creator to make it really "outside the box."

3. Great story line. May not be the same for all but I love LOTR's the books mainly. So deep for me that I like to read them often. You can just get lost in it and forget that you have bills with a 9-5.

The first thing to me is units need to have various resistances to different elements, so that it actually matters what spells you are using. I mean it should matter whether you throw firestorm, or earthquake, or tornado, or cloudburst.

A DD fire based spell should not do much to a fire elemental, maybe even heal it, but a good downpour should dampen it's spirits!

Too many games the only difference, really, is the visual effect but the spells realy are pretty much just a matter of how much damage, area of effect, and duration. So this to me is item number one.

#2 customization as has been stated. MoM was awesome with all the choices you could make at the beginning.

#3 Random events/AI behavior. Even if I make all the same choices, on the same map and with the same opponents, it should unfold as a completely unique game that requiresdifferent tactics to win. Make me respond to surprises, and make enough of them that I won't have seen "all there is to see" after just a few games. It would be great if part of the user generated content was random events.

3. A game calendar that describes the longest games as taking hundreds of years (so you can have things like a faction retreating to a hidden valley to spend a generation or two preparing for war, a la Gondolin in the Children of Hurin).

2. Layers of complexity for micromanagement so you can end up with insane numbers of field units and population centers on the largest maps but choose to manage them with lots of delegation, lots of your own twiddling, or somewhere in between.

1. AIs that make it clear that they're individuals with personal goals that might not always work perfectly with maximizing military efficiency (so you can never be quite sure whether they're likely to do the 'smart thing' or finally give in to their total hatred or love of X and risk everything on a dicey opportunity).

ADD:

x.  The ability to rotate/change angle of the camura

x+1.   A cinematic view that actually makes it feel cinematic

I played Neverwinter Nights regularly maybe 4 or 5 years.  I liked the custom content creation tools, and I think that's why the game has it's longevity (even today).  Elemental needs GREAT world-builder tools so that the player community can develop content over the years.  Exposing the game ruleset, art/media, maps, etc. would be awesome - so that players can create anything ranging from minor rule/item/unit tweaks to total conversions.

I played Empire Earth for years.. Actually re-installed it recently.  What I loved was that it covers such a broad range of eras that you're constantly upgrading or replacing building units.  It satisfies the Build itch.

I played most of the Infinity Engine games (Baldur's Gate, Icewind Dale, Planescape) for years.  I would play them again today if I didn't throw them all out a couple of years ago due to temporary insanity and desire to free up some shelf space.  I played these games because I loved the character generation, the storylines, the scale and scope of the worlds, the large number of things to find and discover, and the happy pants feeling one gets when they level up a character.

I play X3:Reunion.  I would play X3:Terran Conflict but I'm waiting for the future non-DRM release.  I play X3 because I can explore, fight, build and trade.  And I can set units to do these things for me automatically - or I can jump in to micro-manage an individual unit.  I can automate an entire galaxy, and even let the game run while I'm asleep - and have all of it's denizens doing things while I'm away... and then when I wake up I can hop into a cargo hauler, asteroid minor, destroyer, or fighter and do my thing.  It's like the ultimate sand-box + sea monkey aquarium all wrapped up in one.  Elements needs multiple layers of game-play.  I want to build a castle or city, using funds generated from a fairly complex economy, then defend that castle from invaders, while I research and build new advanced weaponry (like composite bows and laminate armor) in preparation for my counter-attack upon the enemy towns.  Elements needs to be multi-layered and free-ranging to the point where it's like it's own universe.

I really like the idea of non-guaranteed research.  I.e. research results are based on expertise in a given area (perhaps with the ability to increase expertise).  MOO3 provided a simliar effect with sliding research: sometimes you got a tech early, and sometimes you got it late.

1. Wide variety in how the game evolves (and therefore huge replayability), for example by having the AI respond and act in very different ways. Here I particularly want to mention random map generation + a sandbox mode as opposed to only a limited number of scenarios.

2. Plenty of different units and spells, cfr. Master of Magic. From what I've seen being discussed here on the forum, both seem to be well under way to be satisfied :)
Also, having different victory conditions -which Elemental is going to have- helps a lot as well here.

3. Character customization and/or various races that are really different (not the classical way of having 3 races with essentially the same units with a different name and very minor differences)

3?    I thought the classical way was having 2 races....  3 is the 'new' thing that is being done to not have only 2....  That was started by starcraft (1998), btw....  after that the bar was permanantly raised to 3 races. (red alert 2 again only had 2, but the expansion added a 3rd)

And rarely do I find in 3 race games are the races the same with different names.  At least 1 of the races is always odd ball.