The Weber Smokey Mountain Cooker is a terrific smoker right out of the box. You don't need to modify it in any way in order to make great barbecue. Having said that, many creative people have developed modifications that improve the functionality, convenience and style of the cooker.
If you're a new WSM owner, try using it as-is for several months before even considering any modifications. By getting to know the performance characteristics of your cooker, by reading the various topics on this website, and by talking with other WSM enthusiasts on The Virtual Weber Bulletin Board, you will develop a sense of what modifications, if any, are right for you.
As you browse through these modifications, you may come across a few and think, "That's crazy! Why would anyone want to do that?" Remember, these mods aren't for everyone. You'll have to decide for yourself which ones make sense for you and the way you barbecue.