Nothing hardware-wise. You need to modify Apple's (logic) board ID checks or else the installer will refuse to run.

First you need to copy the Lion Installer to a bootable USB stick. There's instructions to do this on the web. Next you need to add your machine's board ID to the /System/Library/CoreServices/InstallableMachines.plist and PlatformSupport.plist. Next you need to expand (xar) the Packages/OSInstall.mpkg and add your board ID to the "platformSupportValues" or just modify the checkSupportedMachine to always return "true". If you are using a 32-bit processor, you'll want to modify the "is64bit" function to always return "is64bit". Re-archive the Distribution and Resources back into OSInstall.mpkg again and you should be set to install from the USB stick.