One of the things I find most frustrating about Bronze League is the massive variation in player skill. Take for example the two zerg players in the post below. The first player scouted me, and then successfully executed a cheesy but still valid early rush. The second guy fast expanded, held off my counter push (although with some losses) and then beat me with a proper baneling bust strategy. Both of those guys knew what they were doing.
Since then I've played two terrans. The first allowed me to pin him in his natural expansion. I had total map control, dealing with his one attempt to take a third. He made no effort to break out and attack me - even when I (with true noobish comedy effect) nuked my entire frikking army, he sat there and let me replace the contain before eventually starving him out.
The second terran I played had only played about 8 games ever, none of which he had won, and again he sat in his base and allowed me to lock him in. There was a half-hearted attempt to take a second base, which was easily crushed by the small force I'd left just next to his natural, but other than that he just sat there and waited for me to crack his (not very well defended) base.
I think that my league position in Bronze is probably artificially inflated by wins against these guys. I seem to always beat the real newbies, but it's the harder games where I really need to work and focus.
The problem is that when you start the game you don't know how good your opponent is! The last terran game in particular when I saw zero aggression my mind was racing - I was expecting cloaked banshees, I was checking the islands for hidden expansions, I was dropping and checking for thors (I actually saw an armory, and it turned out later from the replay that he did in fact research the 250mm strike cannons upgrade, but he never built any thors...). In the end though I could have won the game in 10 minutes with a siege tank push, but my worries about macroing properly meant that it took closer to 20 minutes...
I sometimes try to infer from my opponent's portrait how good they are - if they have the medivac portrait, for example, they must be reasonable. That doesn't always hold true though - I personally sometimes use one of the basic portraits on the basis that people might think I'm even worse than I am.