I find that a lot of newbie terran players don't wall off their ramp, and end up struggling to fight off waves of zerglings at the start, for players that have been going for a long while walling off your ramp isn't even thought of, it just gets done. A few marines can successfully hold off the first couple zerg and maybe protoss waves (if they don't have stalkers and roaches etc). Even by the time the other races get their respective ranged units, terran players have got even more marines and begun on bigger things.
Also when new players are going against a terran player by just spawning zerglings because they are quick and cheap (and the 2 for 1 deal the newbies find attractive) they end up being chewed apart by seige tanks and the like, then they quit replay as terran and make mistakes again.
Best way to learn SC is by watching pro play, you don't need to read up on how this enemy beats that enemy, you can easily see the pros countering, unless the person is an idiot and cant put 2 and 2 together they should get at least a basic overview of what they need to do, all thats left after that is getting macro's and timing up to speed.