Interesting Undead Variations
Playing as Undead, you will gather resources in a slightly different manner. First off, you must use an Acolyte to to haunt the nearest gold mine. After you have begun the haunting, you will notice the first major difference about the Undead: Once an Acolyte begins a summoning, he can walk away and begin summoning something else. This means that you have extra time to construct a few farms before beginning real resource and unit production.
A haunted goldmine can be used by up to five Acolytes at a time, who huddle around it and will the gold into your pockets. How that actually works is beyond me, but I am not one to question the power of Ner'zhul. These Acolytes do have to stand at their task in order to keep gold production flowing.
Wood gathering is a bit different on the Undead side of things as well. Apparently Acolytes have an aversion to cutting down trees (or willing them down?). Perhaps the problem is that they do not want to snag their pretty robes. Or, it could be that they are so frail and weak from doing nothing but worshipping a ghost and wishing they were dead in his service (reminds me of high school). Whatever the reason, your Acolytes won't lift a finger in helping you gather wood resources.
Instead, you're going to have to rely on Ghouls to do your chopping and lifting. This means that if you're not provided with a ghoul to start, you're going to have to build a Crypt before you can start gathering wood.
Besides that, one of the most interesting things about playing Undead is using Shades. Shades are a funny invisible unit. It takes a Sacrificial Pit and a suicidal Acolyte to make a shade, but once you have them you can move them anywhere on the map to spy on your enemies. Cool huh?
The last highlight worth mentioning is how fun it is to use Meat Wagons and Necromancers in conjunction. A good strategy is to start off building a few Temples of the Damned. You're doing this in order to do necromancer and skeleton research in one or two of them and train necromancers at the last. While you're doing this, also build Meat Wagons and take them to the Graveyard to begin gathering corpses. (Make sure you enable Get Corpses to run continuously).
Now, here is the important part. Ensure auto-Raise Dead is toggled off on your Necromancers. Proceed towards your enemy with your Necromancers and Corpse-loaded Meat Wagons near eachother. When you get ready to engage, toggle on auto-Raise Dead. Your Necromancers will turn every corpse in the meatwagons into a fighting unit for your side. Additionally, every time an enemy unit dies you will gain another skeleton on your side.
By now, you're probably dying to go play the Undead Campaing, and will know how to keep your Acolytes and Ghouls in check.