As an RTS-inclined gamer, I treat Starcraft and Broodwar with lots of respect, and I can't wait for SC2 to hit the shelves.
It is one of the best games ever made, let alone compared to other RTS's, and is arguably the most well-balanced game ever made. I still play it from time to time and continue to enjoy it as I ever have.
Is it the quintessential RTS, though? No, it isn't. Of course, my answer will depend on my explanation, and some would have a contrary opinion to mine according to my translation, but that's not for me to judge, so on to it, then:
The reason I do not consider Starcraft, or any of the RTS games Blizzard has made so far (since they have all followed the same formula) a pure RTS is that one cannot influence or control the circumstances of one's battles. Compare, for example, the multiplayer modes of Starcraft with those of Rome: Total War. In Starcraft, everyone starts with a set command centre and a set amount of worker units at a roughly equal part of the map. Everyone is equal and starts the battle at level ground with his/her opponents/allies. In Rome: Total War everyone starts with equal amounts of _money_ with which they buy various types and amounts of units and upgrades. Upon entering the map (which is uneven and oftentimes favourable towards a player or a team), every player is assigned an area on which the forces assigned to them can be placed and prepared before the battle even starts. No one knows how the battle will look like before it starts because no one can see the opponent's units being placed before the clock starts ticking.
R:TW's method brings with it a type of map-reading, quick-thinking and versatility that I believe are core qualities of a great tactician and strategist. Upon discovering where one will start compared to one's opponent, the player will have to formulate a strategy of careful frontal confrontation, attrition and skirmishing, flanking and trickiness, or the application of brute force. This does not exist in Starcraft, where most maps are more or less symmetrical and provide equal opportunity for both players. There is less skill and versatile quick-thinking required to win a Starcraft match. The fact that most multiplayer games end within the first few minutes attests to this, as many players know exactly what they were going to do coming into the match. The game becomes more reflexive and base than intellectual and complicated.
I believe a true great strategist must be able to carve out victory from some of the worst of circumstances. There are no bad circumstances in arcadish RTS's such as Starcraft. While there is the argument that starting on even ground with one's opponent narrows down the randomness in a battle and shows that the winner was the person with most skill. I do not challenge this notion as I agree with it, but I believe that in such environments it is not a tactician that is needed, but a man with quick fingers on the keyboard. There's much less creativity involved in a game of Starcraft, even at high levels, compared to the kind one witnesses in Rome: Total War, where you can hide forces in forested areas; take note of enemy unit fatigue; use height to overpower your opponent; take advantage of a morale system, and have a single faction able to bring to bear many types of fundamentally different armies.
Starcraft involves a rudimentary factor of height as a ranged unit will routinely miss an attack at a set percentage if attacking an enemy up a hill, and it does allow some factions to wield somewhat different forces, but I do not believe that this is enough to make a player have to think about positioning and choice of units.
Most of the Starcraft matches I've seen at high level involve players who play roughly the same way; victory falls down to whoever does so faster and better. A battle of skill? Yes, but not of strategy. This also attests to what I've said.
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That said, some people will prefer such a game as Starcraft over one like Rome: Total War, and I tend to waver. The question, however, was whether or not Starcraft is the quintessential RTS, not whether or not it is a good one. That is out of the question as Starcraft is nearly perfect unto itself.