Not saying that these should make it 'the' RTS, nor intending to argue in the detail required to actually settle that, but a summary of reasons fans would give:
- different sides have vastly different types of gameplay. There's no tanking-unit-1 upgrades to tanking-unit 2. Some races are just flat out weaker when it comes to meat shields, or air defence etc. Some builds force you to play without a meatshield, using manouvering and terrain to make up for it.
- one of the first RTS's to introduce 'clock-time' into competitive strategy. Nowhere near as important as with WC3/TFT, where even as a casual but decent ranked ladder player I'd (and most others at that level) would be going in thinking 'I want the main conflict to be sometime between 3min and 3min 30seconds, and delaying through then. If I don't win by 3:30, I'll want to delay until between 6 to 7min). The reason is that different races have different teching times, different importance on teching, and different vulnerability to being pwned while trying to tech.
- did map control very well so long as you aren't playing on high-resource maps (which ruins the game, as it removes the map control aspect)
- one of the first RTSs to make counter-espionage a major part (again, expanded on in WC3/TFT). No matter what race or build, if a half-decent opponent knows what units you have (or sometimes, what buildings you have) you WILL lose almost all the time, so countering opposing espionage units is ultra important.
- different race styles meant that rather than trying for all-round superiority you'd often be sacrificing, say, air-superiority, or standing-fight ability, for advantages in other ways.
- made a slow start towards introducing gamestyles other than 'build, tech, stand-n-fight'. That part was MASSIVELY improved in WC3/TFT, where some races could not win a standing fight against some matchups at ANY stage of the game (e.g. even when Undead were the most powerful race, they could not win a standing fight against orcs at any time of an evenly matched game, nor could they win a standing fight against humans until tier 3, or after tier 3.5). Hence introducing guerilla warfare (eg again from WC3 - undead were actually imbalanced against orcs for some time, despite not being able to win a standing fight, because of their speed advantage with the Death Knight's speed aura. They could consistently run in, take out 1 or 2 buildings, and piss off again without wasting a town portal, every time the orcs left their base to launch an attack. Wasn't balanced again until the orcs gained counter-guerilla buffs, by buffing their raiders so they could use the snare to take out fleeing crypt fiends).
Unlike WC3, SC was highly intuitive to look at. A spectator watching WC3 will have no idea about what beats what, as there as some truly bizarre mixes of armour types and vulnerabilities. Some matchups are truly counterintuitive - upgraded ghouls + destroyers pwning human knights, rifles and casters? Gryphons not being able to damage destroyers? Mass wyverns pwning mass gargoyles despite gargoyles having insane air-to-air damage and a handful of gargs being able to pwn a mid-size flock of wyverns (there's a critical mass where the focus-fire from wyverns gets too much). Mountain giants being so low damage per food that you're best off literally ignoring them while killing others? Mass frost wyrms sucking, but 1 or 2 being a battle-changer vs orcs (but not against anyone else? WC3 had an absurd learning curve for those who wanted to 'hop in' to ladder matches. To get even a mid server 'level', let alone a really really low 'ranking' on a server you basically had to learn the various damage types vs units, then learn the cookie-cutter strats to get the first few levels, and then learn to ditch the cookie cutter to get ranked from there. Not much fun for someone wanting to just fire it up and play Battlenet.
SC was the exact opposite. If something LOOKS like it will counter something, then it will. No orc casters able to go toe-to-to with melee. If it looks anti-air it will kill anti-air. If it has 'tank' at the end of the name, it will pwn anything on the ground, and nothing else. If it looks fast and flimsy, it will be fast, do decent damage and be flimsy. SC you can be really quite decent at without ever studying a 'unit armour and damage vulnerability sheet' in your life. I hope they learnt their lesson and keep that intuitive 'feel' (where it's clear what is capable of countering what - even if it is left unclear whether it's a soft or a hard counter - rather than making no sense outside of a spreadsheet).