1) Glad you decided to not build a laptop, as it's a completely different beast than building a desktop PC. We'd love to advise you to build a desktop, but laptops, leave it to the manufacturers unless you got personal experience/contacts on how to build one.
2) Decide exactly what the purpose the laptop is for, because upgrading/replacing parts is a bigger pain in a laptop than a PC. For example, you want a bigger screen? Got to buy a brand new laptop for that whereas a monitor is easy to replace.
3) Warranty means a lot more on a laptop so pay attention to what manufacturer's offer. If you're cautious, or think you can experience problems with it, it sometimes does pay to buy an extended warranty, but that's your own personal preference.
4) Switchable graphics vs Dedicated graphics? Dedicated cards use up more battery life, but are usually faster performance, gamin wise because better cards are used. Switchable graphics are still great and can give you great FPS while at the same time giving you the added flexibility of turning the GPU off when you're only web browsing or answering e-mail. However, the option may not be available on the GPU you want to use.
5) Look at reviews of the laptops and try to test them out yourself, such as in Best Buy or Microcenter. One of the biggest complaints I see is about the screen. Other complaints would be about heat dissipation, size of the battery, the keyboard, and even the chassis design.
There's some basic tips for you on looking at laptops.
