Unless there is a mac specific version of an app like djay, then you'll not normally be able to run iphone/ipad apps on a macbook. You can backup and store all the apps on your macbook, but you'll not be able to run the apps. The "Mac App Store" is getting better all the time, with lots of great apps both free and paid, available from the app store icon on the OS X Lion desktop that will come with your new macbook.
I switched from Windows to Mac, about 4 years ago and it was the best move I ever made, the mac "just works" without all the hassle that windows brings, although it does depend on what you use your computer for.
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