I've played the beta and am a pretty
good player. If there is one thing I've found to be critical to
playing well it is choosing the right race. To do well you shouldn't
pick Terran because they are human, Protoss because they are cool
and high tech, or zerg because of their monstrous appetite.
Each race has a distinct play-style and/or play-styles unique to
their race. Each player has a play-style that best suits their own
personal play-style and play level. To do the best you can you need
to find the play-style that best suits yourself, and then pick your
race accordingly. Here are the races and how they play.
Terran:
Terran: Terran is an extremely defensive race. The race
centers around creating a heavily fortified position in the early
game, usually by creating a front door wall with supply depots,
barracks, and/or factories. The Terrans than build up a large force
unopposed and move out when the time is right. Because Terran
strategy is very straightforward and in the process of climbing the
tech tree they unlock almost everything Terran is generally
considered the "Easy" race.

If the Terrans have a weakness it is that they can only operate as
many bases as they can defend at one time. Terrans are usually
limited to only 1 or 2 bases. Their units are slow and largely
dependant on positioning. They are superior in a chokepoint but weak
on open ground.
Zerg:
Zerg: If the Terran race says "This is my base and you
can't do anything to stop me from holding it" the typical Zerg
response is "Ok, but I get the other 90% of the map". Zerg gets a
large number of extremely mobile units. Speed upgraded Zerglings are
lightning fast, mutalisks are fast and flexible, Roaches and
infesters can travel while burrowed. The Terran fight in a
chokepoint, like near their base, and the Zerg fight in the open
ground of the center of the map.

The zerg win by out-producing, or out-macroing, their opponent. Good
Zerg players usually fall into one of two categories. There are the
players who build almost nothing but drones as much as they can, for
as long as they can. At any one point they have just enough forces
to survive for now, and they build much of their army after they
spot the enemy army leaving their base. They build nothing but the
best counter for the opponent's forces.
The other strategy revolves around taking an active part in
preventing the opponent from being able to macro. These players use
their high mobility units to constantly harass their enemy. Often
attacking the opponent's workers with mutalisks and other surprises.
Instead of bolstering their own economy they build fast units and
stop their opponent's. Constant harass, mutalisks, baneling drops,
anything to slow the opponent is standard.
If the Zerg have a weakness it is that all of their strategies are
highly dependant on the player's skill. A macro heavy zerg needs to
know exactly how large an army the opponent could have at any given
time, and exactly how much of an army he will need to keep to stop
that army. A harass heavy zerg needs unparalleled control over his
units to keep them constantly moving and attacking. The zerg is
often considered the "hard" race, BUT if you're willing to work with
their play-style it can also give great results and be very
rewarding at the same time.
Protoss:
Protoss: There is a reason I'm covering Protoss last and that is
because the Protoss race is best compared side by side with the
other races. The Protoss can play very aggressively, using warp in
and a large number of gateways to quickly drop a strong force on the
enemies doorstep. A front door closed by gateways and a zealot or
two at the entrance can be difficult to break.

The Protoss can put up a strong defense compared to the zerg, but
not nearly as tough as the Terrans. The Protoss forces are more
mobile than the Terrans but can expect to be outmaneuvered by the
Zerg. The Protoss are strong against the zerg in a tight chokepoint,
but should fight the Terrans on open ground. They can expand to a
new base early, or live off of 1 base for some time.
The Protoss doesn't really have a definite weakness, but it doesn't
specialize either. Its better than zerg at things zerg is bad at and
better than Terran at things Terran is bad at. Its a very well
rounded race in general. Protoss are generally considered to be the
moderately difficult race as the race doesn't require quite as much
micro and knowledge as the Zerg, but he still needs to know how to
be effective against an opponent using a given strategy.