I'd settle for nothing less than totally realistic weapons system and system modes with supporting procedures such as MRS updates and maybe boresighting modeled extremely accurately. Also, contrast, gain, focus, etc. for thermal and daysight optics would be nice as well. Realistic ballistics are a must, scratch that, given, a long with extremely realistic damage for modules such as GPS, TIS, LRF, ammo rack etc. An ability to connect to DCS would be nice, and DCS CA would probably be the engine for such a sim, giving it built in air support. Destructible environments... well, for a CA-attached sim, ask the developers of that. For an indie sim, such as America's Armor, hell to the yeah! This includes aesthetic damage on the tanks, correct? The maps... Black Sea is big enough, indie game's maps would probably have to be gigantic for me to be remotely impressed

. Also, forgot to metion, 3d interiors are a must, loading animations would be nice. Also, if the tank in real life is equipped with datalink,( BOWMAN, Challenger 2; IVIS, M1A2, A2 SEP )it would be excellent if that were modelled as well.
I disagree with this in many respects.
Total realism = niche product that a handful of people will be playing 3 months after you release it. Are you gonna make players wait 45 minutes if they throw a track? That's realistic. Hell, make it more realistic by forcing the player's avatars actually fix the track in that 45 minutes.
You need to make it accessible and fun, even if it costs you some realism.
World of tanks, for example, is hugely popular. I'm willing to bet that even with it being free to play, they make money hand over fist. Thousands of people play it because it is so easy to play. You don't have to go as 'accessible' as they are (and in fact, I really wouldn't want that) but if you go totally opposite and make people do this 100% realistically.
Also, 3D interiors seem like a massive waste of resources in my opinion. For all the effort you put into making an interior, which you're not looking at if you're trying to fight in your tank, your modellers could be making other things, like scenery, targets, damage models, etc.
For me?
I'm interested in game play. Specifically single-player.
I think the Third Wire series could be a good guide for a tank game. Realistic enough, but doesn't require you to learn an entire manual to play.
Extremely moddable, with a dynamic campaign (not Falcon 4 detail, but something with replayability) and crew management. Panzer Elite was an excellent blend of realism and gameplay (though the missions were canned - my biggest beef with the game) and would be a good model for any tank sim.
If you're doing WWII, I'd like someone to develop a good method for tank-infantry cooperation. So many tank sims/games focus on tank-on-tank combat. The vast majority of tank combat in WWII and, hell even today, was in support of infantry. I think a good tank game would make an effort to improve on this.