One of the fundamental things that makes the Ork, er, Orky is the Waaagh special rule. Nothing says I'm here to kick your ass like charging towards your enemy windmilling your arms and dribbling incoherent expletives! The Waaagh is key to making Orks work.
The number one thing that is on your side as an Ork is weight of numbers, the number one thing that is against you as an Ork player is your opponent's shooting whitteling down your numerical advantage. You might say “But I can take fast transports! He's not going to be able to shoot me up too much”. Well you're on the right lines but you can only cram 12 Boyz into a Trukk so you get to the enemy lines fast but you've lost your weight of numbers, 12 Orks just aren't that effective, the same is true of Biker Boyz. Nob Bikers are an exception because they don't really need the same weight of numbers as Boyz do but if you play Nob Bikers no-one will like you and deep down you'll feel bad and a little less happy with you life in general.
The solution to the problem is Battle Wagons and a good old Waaagh. 20 Boys crammed into a Battle Wagon allows you to bring weight of numbers to bare and, with a little luck (or Ghazkull), nowhere on the board is safe from a turn two charge from your mean greenies. What this means in real terms is that, at worst your opponent is only going to get two turns of shooting off at you, one turn if you went first! And this means you are going to be able to bring significant numbers of Boyz to mess up his lines.
The way I run this is to have 2 mobs of Boyz in Battle Wagons to act as a first strike, a way of tying up shooting units as soon as possible to allow my main block of 30 Slugga Boyz to reach my opponent's lines relatively unscathed. 20 Boyz in a Battle Wagon are going to do some damage but are unlikely to win out easily in most combats. However, 20 Boys followed closely by 30 Boyz is seriously going to mess up anyone's day and I find it an effective way to play Orks.
The last thing to say on the Waaagh is to mention the Prophet of the Waaagh himself; Ghazkull Thrakka. Ghaz is a total monster on his own but his ability to guarantee you a 6” fleet move on the turn you Waaagh is just golden. When you really need to roll well to ensure you make it into the crucial turn 2 combat, having an assured 6” move in your pocket is just too good to underestimate. There are many other reasons to take Ghaz in your army but this one is critical to many a game plan and no opponent of Orks should forget that!