This special version of Metal Gear Solid adds first-person view, a level of difficulty for newcomers, bettered artificial intelligence for some enemies (including bosses) on higher difficulty settings, and, on the third disc Metal Gear Solid: VR Missions.
The PC version includes an expansion disc originally released for the Playstation as Metal Gear Solid: VR Missions which added over 300 missions set in the VR training environment of the original, the ability to play as the ninja, and a "photo shoot" mode featuring the game's female cast among other goodies.
Also, the PC version was supposed to have new hi-res texture skins for all the models (especially Snake himself), but Konami axed the idea.
Developer Dimiter Stanev:
From the only phone talk we had with Kojima, through a translator we were advised not to use high-res textures, since there was no good eye animation for the cut-scenes - e.g. higher texture fidelity would ruin the effect a blurry one gives, since you can't focus your own eyes on it. With a high-fidelity texture it would've looked as puppets, dolls, etc.
I admit that this by far, is the most important lesson I've learned from anyone in the game industry - sometimes less is more, and wrong is right.