Great music.
The things that really stood out for me in the original Tomb Raider were the organic and realistic approach to mechanics. There was basically no HUD, Lara would just aim at what's in front of her, today everything needs to be marked, pointed out and highlighted with crosshairs and icons. Sure, this really needs to be sometimes in some games, but in TR you were seeking for information about the world in the world itself, that is great.
Also the physical element stood in contrast to the static overly effective movement system of, let's say the Quake games. If pushing quads Lara needed to move back, take a short break and then push again, until job was done. When she'd grab an edge, she wasn't
teleporting up, but took some time to gain strength and pull herself up. Stuff like this added to the gameplay immersion. When I look at that new Metal Gear game, the character hops around like a Moskito on dope, disgusting.
The art of pace was great in TR. It had fast pace and slow pace in the right moments.
By the way, the first three titles are available on GOG.com:
http://www.gog.com/gamecard/tomb_raider_123
GOG games are usually optimized to work on modern machines, so it's a good chance to play them again. I'm going to grab that at some point.