One of the reasons that the genre is so immensely popular is that the games that exist within its boundaries lend themselves to an extreme amount of consideration and study amongst the hardcore audience of players. Much like certain first-person shooters like Quake, Quake 3, and Counter-Strike attract a very dedicated group of hardcore players who memorize the layouts of every map and can absolutely thrash even "very good" opponents in tournaments with ease, certain real-time strategy gamers gravitate to specific titles that are particularly conducive to gameplay which rewards a deep, occasionally disturbing, level of understanding of the game mechanics. The game that immediately comes to mind when thinking about the genre like this is no other than Starcraft, which, particularly in Korea, has an overwhelming number of gamers which treat the game like a religion. I've heard rumor of Starcraft tournament stars being likened to the traditional rock stars of American culture. Whether this claim has any legitimacy is unknown to me, as I haven't really traveled to Korea lately, much less a Korean Starcraft tournament.
For me, the appeal of the genre doesn't quite stretch to the hardcore understanding that would lead me into victory in any tournament. My favorite RTS is, without a doubt, Rise of Nations. I feel that this game is the perfect blend of typical RTS research-and-attack conventions while also having a very unique and interesting economy. I was never a much of a heretical fan of the Age of Empires series (though I have enjoyed them greatly) and, for me, Rise of Nations a fantastic middle-ground between Civilization and more action-oriented RTSs like Command & Conquer and Warcraft that Age of Empires slightly missed. In the early parts of the game it was necessary to build up the area your central settlement and explore the continent you were spawned on while also researching your way into the next epoch. At some point, you'd expand your settlement (and this was a necessary step unlike a game of Warcraft 2/3 where an expansion base may not be the best idea) and then start establishing trade routes, building up your borders to ensure a strong defense when a foolhardy opposing tribe thought they could break down your walls and lay waste to your settlement early on in the game. Each game had a truly epic sense of scale as it took you from spears to fully-automatic guns, bazookas, and a staple of any good game: nuclear weapons. The game also had alternate victory conditions that didn't necessarily rely on violence (thought you would, almost definitely, engage in a handful of skirmish). Most importantly, though, this game did not require intense micromanagement for the most part -- though, in the late game, effectively managing all of one's settlements was an absolute pain -- and managed to contain all of its gameplay in a timespan under two hours which, for me, is probably when my attention for any one gameplay session begins to wane.
As I think about it now, the reason that Rise of Nations captivated me the way it did was due to its superb ability to pack all of my favorite things about turn-based strategy games into a real-time strategy game progression. Some games I loved playing an aggressive military game while others I enjoyed playing defensively and researching my way to victory while other times I enjoyed doing nothing but researching my way to nuclear weapons and blowing the rest of the map to radioactive wasteland. What it all boils down to is that real-time strategy games work because they promote a certain level of decisiveness in the gamer that can be reflected in-game surprisingly quickly whether it be related to a research choice and the immediate and long-term consequences, a choice to position a tank in combat somewhere specific or to put extra money into artillery instead of a tech which would boost your economy, and so on.
RTS games are, fundamentally, a microcosm of real-life topics and gameplay mechanics. They contain real-time combat that is dependent on tactics both big picture and instantaneous decision, balancing an economy that must fund both research and military, defense of your home base(s), exploration, and expansion. Sins of a Solar Empire, for instance, is a relatively slow-paced game that has its foundation in a lot of 4X (eXplore, eXpand, eXploit and eXterminate -- why it's not a 4E game has always boggled my mind; I guess X is just in a more extreme segment of the alphabet) turn-based gameplay that puts as much emphasis on economy and diplomacy as it does combat. In the end, Sins is primarily won through combat but the intelligent player is able to leverage a particularly strong economy to form alliances with other players in a multiplayer game who can function as the sword to his savings. Company of Heroes, on the other hand, is a game which is firmly rooted in combat. It still gives players a variety of possible play styles (I like to play infantry- and artillery-heavy) but the reason the game is a success is because it places such a fine point on its combat that, instead of focusing on large-scale economy decisions, the choices the player has to make are now large-scale combat choices -- how to capture territories and key points in a way that, if attacked in the process, will result in the most damage to the opponents forces and the least to the player's. So a player could rush into a point with a single piece of armor and a pair of infantry forces to capture the point while being supported by heavy machine gun fire, mortar shell launching, and the ability to call in a precision artillery strike if a hasty escape needs to be made.
The possibility for a large-scale RTS which manages to seamlessly mix the large-scale issues of research, diplomacy, and economy of a player-driven empire along with the less time-intensive and more short-term rewarding nature of visceral RTS combat is an idea which I absolutely adore. The day a game like this comes about is the day that RTS gamers can have their life-ending World of Warcraft.