Modern Laptop:
Multiple core processor, at least 2GHz per core (Intel i3, i5, i7; AMD Phenom)
4-8GB RAM
200+ GB Hard Drive (eSATA)
BluRay / DVD-R/RW drive (with lightscribe is pretty standard these days as well)
You can buy one that says Apple, HP, Dell, Acer, Sony, and a few other brands. You'll pay more for the Apple, but some people like to pay more and like Apple's Operating System.
I prefer Windows 7, but that's me. I cannot seem to get used to Apple's OS. I think it's clunky and incredibly unfriendly. Of course, I never took the time to learn it. I had a Mac, but got rid of it after a week. I couldn't do 90% of what I normally do on a Windows based computer.
Thing I remind people is Apple is incredible proprietary. Generally the only software that can go on a Mac is something pre-approved by Apple. With Windows, I can install whatever I want and a lot of what I use is freeware. Even software that is compatible with each OS, the Mac OS version is generally more expensive than the Windows version. Apple products will always have you spending more money.
Of course, all of that is what people tend to hate about Windows...it's too open for the user to make changes that cause it to crash (because people go to the wrong web sites that install crap on your computer - be aware, they're targeting Macs now due to the increase in Apple's market share).
If you want the best of both worlds, buy a 17" MacBook, buy Parallels, buy Windows 7 and run a dual boot laptop. You're going to spend $3000 to do it, but you're guaranteed to run anything you want.