Someone I know claimed that SC2 is entirely counter-based (and will stay this way forever) because of two things:
1) Units build faster
2) There are no "solid" compositions that are good against everything.
Now, I'm not sure what #1 has to do with the question. Faster unit build (for Protoss especially) does mean specific harass strats are easier to shut down (by building the counter unit to, say, Banshees), and I guess it could also mean that if you win a battle really badly you can bring in specific counters to mop up the units that are left? But I really don't understand how build times affect the question at all. Yet, if your opponent only has tanks in BW then it's easier to counter that (because lings build ridiculously fast) than it is to counter m&m since you need muta, lurker, defiler, or some combination. But I don't think this is the key to the problem. Maybe someone enlighten me?
#2 though, I get. Or at least I understand the perception that leads to the claim. What I don't think people understand though is that it is (probably) entirely a matter of game history: the beta's only a few months old. Players haven't had time to work out balanced compositions yet, and Blizzard hasn't had time to observe the game and adjust balance for that consideration.
Consider the original Brood War beta: people went m&m in every matchup, tanks were (only?) for support and drops, and vultures even rarer support - or at least that's the idea you get from reading the old battle reports. Even when BoxeR revolutionized Terran, at first he was still using marines in those dropships to wreck stuff. Protoss had the hard counters here: storm and reavers > all. And that might be why Protoss dominated the early days of Broodwar: they came out on top of the counter battle every time.
This isn't to say people weren't experimenting. Some dude named Zileas made shuttle-reaver micro famous right off the bat, this wasn't a weird discovery years later. People were playing with early expand builds, mutalisk harass - if you want a look at the early days of Broodwar, go read through the BattleReports.com early archives. (It's also a website well worth your time and effort if you can tear yourself away from TL. ) All sorts of stuff going on, and lots of rock-paper-scissors counter thoughts, but not much in the way of pre-determined cut-and-dried build orders and today's set metagame and pre-determined build-order play that only artists like BoxeR and certified nut-jobs like Horang2 can disturb.
Now consider the SC2 beta. We have people trying all sorts of weird stuff, from roach-only to no-roach Zerg builds, trying to make Phoenixes work, "cheesing" with warp gates and reapers, going straight marauder with minimal (usually ghost) backup, etc. etc. There is no such thing right now, as far as I can tell, as standard play in the sense we have it in BW. No 1-rax expand builds, except so far as we could do it in BW and it seems to work here: but timings haven't been perfected. Very little aggressive /army/ usage, despite the prevalence of harass. Very few attempts at map control via army presence, meaning less chance of battles testing your compositions. Most battles (from what I've seen) are straight up base attacks, which tend to be either inconclusive (positionings haven't really been worked out yet) or vastly favor one opponent often leading to a straight GG. In short, we haven't yet done the SC2 equivalent of proving that m&m doesn't work well against Protoss unless you're timing push stalwart fOrGG. (Of course, small maps are playing into this a little bit, just like they did in the BW beta.)
What I'm seeing right now in SC2 - especially from people who are complaining - is a thought-process that would look like this if applied to BW (and looks a lot like some of the stuff you can imagine happening in the BW beta):
Game 1:
A: "Well Marines have great DPS and with Medics they're durable so I'm going to try to make that my base."
B: "Ah, marines, let's mass up reavers to counter."
A: "Hmm, he's got reavers out, I need more range, Tanks!"
Tank + M&M rolls, Player B ggs and goes on the forum to scream about Terran imba.
Game 2
-repeat game 1- but when tanks come out:
Player B: "Wait, if I pick up the reavers with shuttles, they can stay alive longer!"
Player A gets massacred this time, and goes off to whine about reavers > all.
...and we could continue this through several more steps, but you get the point. It's going to take months (maybe years) of play, several patches, and a couple billion games of Starcraft 2 before we start actually getting to the point with this game where we know what are the "safe" builds and "strong" unit compositions.
So in the meantime, chill out! - and pardon my ranting.