The main problem with going from the campaign to the multiplayer is that it's a completely different dynamic... in the campaign, typically you start with a very small base, while the AI has a large sprawling complex. However the AI doesn't grow and expand much, or build much of an army except for the occasional attacking waves, or replacing losses. So the right move is to turtle up, build up a huge army, and then go on the attack. And if it takes you a while to build up that army, well, that doesn't matter too much. While in the multiplayer, you start on even footing, and both of you are building up and expanding your bases, at the same time. Which means that attacks are viable much earlier, and if you wait too long to attack you'll lose. If you turtle up longer and build a larger army before pushing out, it only means your opponent will also have built a stronger army and will be harder to beat.
Similarly, things like harassment, containment, and cheese like proxies and rushes, don't really make any sense in the campaign, but are important parts of the MP game (even if you don't do them yourself, you have to be ready for them from your opponent).
Which isn't to say the campaign is bad in any way, it's still very fun. It's just a very different game to the multiplayer.
While no one overhear you quickly tell me not cow cow.
but how about watch phone?