First 0ff - the SC2 game is amazing. I haven't done any Battle.net games yet, but the storyline is really great. I like the changes they made to the briefing room, but I will concede that it's just another RTS. If you liked the original, you'll probably like SC2. If you hate RTS games, this isn't going to suddenly make you a convert.
Oh, and there ARE Firebats in the single person, which made me a happy panda. I don't mind them not being in the multi-player since that isn't my focus. It's almost like an ability is available for PvE but not in PvP. What a twist!
On to the real issue - combat tactics
Now, I'm no master of Starcraft tactics - especially when placed against another human opponent. Much like all video game to RL conversions, certain things just don't mix. For example, I've cleared rooms and rescued hostages in real life scenarios (training mind you) - but I suck at Counter Strike.
Regardless, in combat scenarios there are a few generally accepted methods of winning any battle.
This is a tactic that is widely used by the US military, that unfortunately isn't as effective as it could be. What I'm talking about here is the sniper kill, or in Starcraft - the surgical nuclear strike. Now while I've never been overly successful with a nuke strike in SC, it is a great tactic. In the game it's extremely expensive, and it takes so long to land that an alert opponent can usually flush your Ghost out.
If you can pull it off though, a surgical strike can be a devastating, or even crippling move. It's not even necessary to kill off troops or even important building structures. Somtimes it's enough to just conduct...
The spoiling attack
This is the strike that will be most likely to drive your opponent to hair pulling frustration. Spoiling attacks aren't designed for a high level of destruction, or even actual destruction. What you're looking to accomplish is to simply throw off your opponents timing or momentum. Take out all of an opponents worker bees, and suddenly he can't afford to build troops. Resources that would be spent arming his forces are instead spent trying to get back to his original level of production.
These attacks can also stall your opponent, as they have to adjust mentally to the change in their plans. While this won't throw off a master strategist (they're already four moves ahead of you), it will work wonders against that one-trick pony player who's been beating you.
Don't focus on full destruction though. These attacks are the bread and butter of the guerrilla warrior. In, out, gone. You don't want to get your own troops caught in the firefight, so fast moving troops are generally better suited (or an airdrop). Remember, this attack isn't intended to dislodge them completely from an area. Instead you want to force them to spend more resources on a secondary target. Of course, this is all in an effort to set them up for
The 2x4 between the eyes
This isn't a zerg swarm, a marine rush, or any other type of fast attack. This is the sound of a dozen siege tanks opening up at once, eight Yamoto-guns (are these in SC:2?), or a full swarm of Valks on your air force. This attack makes Pearl Harbor look like a field trip to the petting zoo. It is the culmination of your forces, bearing down on that weakest point.
Forget Waterloo, Troy, or the Battle of the Bulge - this is Hiroshima and Nagasaki rolled into one giant attack. This single assault can be conducted by an overwhelming force or a significantly powerful one, will push your opponent to the wall. Where you hopefully have someone hiding to whack him in the kneecaps.
Personally, I like to use a two or three pronged assault if I can - especially if I have a partner I can coordinate them with. When your opponent is responding to an assault on multiple fronts, you can split them open like a ripe melon. WWII would have turned out a lot different if Hitler hadn't opened up a third front (Africa, Europe, Asia) with Russia, as he would have been able to throw a significantly larger force of troops against the Allies in Europe.
In the end
You just need to mix up your attacks. Stay fluid. I personally like to reference a jellyfish here. You need to be mobile enough to respond to any change in pressure from your opponent, while still fielding attacks of your own. Never let the enemy work unhindered. Even sending small units in to harass and confuse will take his attention away from the main task - obliterating you.