The AI good but not awesome. If you practice a lot you will find out how mediocre it can be.
And when I say a lot I mean like 1000 hours a year. Those nutjobs you see online that execute 300 actions per minute spend a lot of time practicing. Once you get good at Bnet the single player campaign will seem like a joke. Same with Starcraft.
Watch some training videos on youtube.
http://www.youtube.com/results?search_query=warcraft+3+training&search_type=&aq=f
One of those vids says "training for newbs".
BASIC TIPS FOR ALMOST ALL RTS's!
1. Lots of resources gathers. Build as many as you can as fast as possible. The slow ponderous resource gathering often found in campaigns will get you stomped online.
2. Make one basic fighter building ASAP. This is normally a barracks or similar structure.
3. Make another barracks! Training one warrior/soldier at a time is usually an easy recipe for failing.
4. Heroes or special units are often excellent deals for the money, and add easy power to your army.
5. Upgrades can normally wait until you have a stockpile of resources. For the money they are usually not too helpful, not as much as a unit. There are some exceptions in some games, where an upgrade is practically required for your army to be any darn good at all. Age of Mythology has a couple of these for each race. But rarely are "+1 attack" or similar upgrades a good idea. The useful ones are those that add whole new powers and abilities to your units.
But in general upgrades can wait. But when you do decide to research them its best to have multiple buildings so they can be cranked out quickly. Dawn of War is a perfect example of a game that mixes low-cost but useful upgrades with expensive and mediocre upgrades.
6. Ranged is normally better than melee unless the melee is MUCH more powerful. If you've ever played a Tower Defense game you know that excellent range will often let you attack earlier and keep attacking longer. RTS's are usually the same. A short ranged attacker has to endure punishment before he can even make his first strike. And in a messy battle the ranged folks will often get tons of free hits while everyone else staggers around.
I had a hard time learning this in Starcraft. Hydralisks are infinitely superior to zerglings. Even for the money they are a much better deal. Zerglings are OK as a distraction or speed bump while the ranged attackers do the real damage. Same for Ultralisks. They are mostly good to keep the enemy occupied but wont normally do significant damage in a battle. This principle also applies to Marines vs Firebats and Dragoons vs Zealots.
Again, if the melee character has a special ability or is just super powerful, it may negate the ranged units advantage. But those units tend to be very expensive.
In Dawn of War the melee damage for all units is normally much higher than their shoot damage, and melee-only fighters tend to be massively over powered. Giving a space marine a flamer is OK, but the short range means he will have to take plenty of hits before he uses it. Upgrading him with a heavy bolter (machine gun) is often more useful, even if the enemy eventually moves in close.
7. Special powers are often worth the micro-managing of frail or important units. The best abilities are those that stun the enemy or slow down their attack or movement. Again, that means in a large battle they probably wont be able to hit as frequently. Special powers that do straight-up damage are often not as useful over the long run, unless they can slaughter many units at one time.
Healing special powers vary game-by-game. If they work automatically and dont cost anything and can be used at a short range (versus standing right next to the unit) then they are alright.
In Supreme Commander it is often easier to make a new unit than repair an old one. Repairing usually takes a long time and normally requires active input on your part. In Age of Mythology its automatic if a unit is within range, and it also works quickly. Each race (Atlantean, Greek, Egyptian, Norse) has their own way of healing but each can be effective if used wisely. The Egyptians are the best cuz the healer is also your basic hero unit, and cheap to produce. They can travel along with the army and heal when needed. They do it automatically and have a good range too.
In Starcraft each side is very different which adds to the strategy. Terrans cant heal except in the expansion pack and only with a medic. As I recall, the medic needs active input to use her healing properly. Their building will actually burn up and explode if damaged and not repaired eventually. The zerg heal naturally, but slowly. That can be infuriating if you are able to micro-manage well. The protoss only heal their shields and that requires micro-managing. But the shield can be put up instantly with a shield regen, and its normally much more powerful than their health (which cant regen at all.)
8. In most games defensive structures are not a good use of resources. They are expensive and need micro-managing to repair. They are often just speed bumps to an enemy who wants to destroy you. Same deal with weak melee units as mentioned above. Even if they are cheap they are still a bad deal for the money. The exception to this is walls, which usually get built fast and absorb tons of damage. Having ranged units on the other side is great for pelting the enemy while they figure out what to do. If the game doesnt have walls or doesnt have good walls, often a cheap building of another sort will work. In Warcraft 2 this was farms. You only get so many population points from a given number of farms, but their relative low cost and high hit points made them into pretty darn good walls for your base. An exception to the defensive structure rule is Total Annihilation and Supreme Commander. In those games the medium and light lasers were very helpful in base defense and pestering an advancing army. The anti-air towers were FAR more useful than anti-air units, and essential in base defense, especially versus bombers intent on hitting your power supplies. The shield building were also extremely nice. Same with Act of War. Anti-tank is helpful and sometimes required. Anti-air is a must. And as cheap as they were it was often better to build a row of towers instead of a fleet of vehicles.
Also, any game that has a fortress of some sort can generally follow the rules of defensive structures. Its not really useful unless its quick & easy to put up or has massive firepower. However sometimes fortresses can also make units or special units, and then they are more valuable than a normal barracks. The fortresses in Age of Mythology are like this. Relatively cheap for their power, quick to build (with plenty of laborers on hand) and they made the best cavalry in the game.
9. Practice. The most important tip. In Starcraft and Warcraft 3 you can make custom maps. Make one that lets you ignore enemies for a while and practice your base-building technique so its as fast as possible.