From Poxous of WPS/Direwolf Fame:
Building an army around several units is always a better idea than around one. The trick in a GT is to be ready for everything and not ride in on a one trick pony.
Think on these terms:
What will I do vs all Knight armies?
Can I survive 10+ powerdice?
Can I survive 40+ shoots + war machines?
Steam Tank?
TKs magic?
Can I catch WEs?
4-5 chaos chariots?
Fear/Terror?
Spirit Hosts?
Fanatics?
If you're going for the total win then you have to address each issue in your army comp. Some times the most stupid looking units can do wonders.
And from Warlord Merkit:
I will say that you should consider how to deal with the following types of armies.
1) Large terror causing flyer and/or foot (esp. drakes)
2) Heavy (10+ power dice) magic
3) Lots of cavalry
4) lots of shooting
5) the horde
You will not encounter all of these in your tournament games, but I think these are classes of armies that will be pretty popular.
Both Poxous and Merkit know that a Tournament army must:
1) Be able to deal with a wide variety of opponents - although you will not see every possible piece of nastiness in a 4-6 game tournament, you will almost certainly see one or two of them. If you are not prepared to meet them, do be prepared to take the loss.
2) Not be overly slanted to one aspect of the game - Especially in the US, tournaments tend to use a variety of scenarios. These scenarios are most often conceived with the intent of penalizing a certain phase of the game, normally magic or shooting. If you depend on these too much, it's very difficult to win that game.
I would also like to add a couple more points ... a Tournament army must:
3) Not be overly dependent on dice rolls - The longer a tournament goes, the more likely it is that you will suffer from the law of averages in your dice rolling. You will have a great game or two, but you will also have a horrible one. If your army depends on ranks and banners to win games ... these aren't variables - you will have them every game. If the army depends on causing casualties to win, just be aware it won't always roll your way. This is actually where the Skaven horde army excels - they don't cause many wounds ... but they always have ranks and banner ... and usually a flank.
4) Be fun to play against - Sportsmanship scores win and lose tournaments. You can build the uber list of ultimate slayage ... but I doubt you'll win a tournament with it. Now, I am not saying to build in a weakness or to make a "fluffy" army. But remember that the folks you are playing against have dropped some money (sometimes quite a bit) to come and push little army men around a table for the day. Most folks want to get in some combats, and to feel that they actually played - so play them. Set up your army to confront them ... heck, even bring some sacrificial units that are meant to die. This is actually where Wood Elves have the most problem ... people hate chasing them around the board.
Anyway ... just my thoughts ...
Jeff