An amazingly well-made and highly detailed game for its age.
First off, the details. This is the only game I think I've ever played that pays attention to where you're hurt, where the enemy is hurt, and how that would affect movement. I. E. your fifty ton battlemech is shot in the knee, and it will limp. You shoot an enemy in the knee, and it will trip and fall over. You can fall over, too, which is interesting - the fall motion is not scripted. If you start falling, you can save yourself - just take a step back and you can steady your mech. Your mech will also react differently depending on where it's hit, and will recoil and rotate to the side you fire from. Lasers have unlimited ammo like they should, and generate a lot of heat. Weapon ammunition is freaking heavy, which is something that often isn't taken into consideration. You get shot too much in one arm, the arm will get blown off and you'll lose any weapons that were mounted there.
Second, the customization. While MechWarrior offers nowhere near the level of visual customization that something like Armored Core does, it still offers a solid variety of mechs - ranging from super fast featherweights to ultra heavy mechs that are nigh impossible to kill - and an insane number of weapons. Now I understand that many weapons are reiterations of each other (such as large, medium, small, large extended range, medium extended range, and small extended range lasers, and a CLAN version of each of those) but for an old game it's still amazing. There are two types of missiles, long range and short, "Autocannons" which are basically burst-fire machine guns, and machine guns, as well as Gauss Rifles, Lasers, Pulse Lasers, and Particle Projection Cannons.
Third, the gameplay itself. When I played Armored Core, I was treated in the lap of luxury. Sure my mission reward had to pay for my ammo and damage, but at least all my ammo came back for the next mission. MechWarrior, any armor or ammunition you use/lose must be scrounged from the wreckage of your fallen enemies, unless you wish to use solely lasers (which can work). After each mission, you can Allocate your salvage, picking and choosing what you want to keep and what you want to leave behind. It teaches you "the true meaning of survival," to quote a friend. Then there's the whole "MFB" or Mobile Field Base idea. You start the game with three. Each can hold 300 tons of weapons, ammo, and giant robot. Now the total 900 tons is a lot at first, but you can fill it surprisingly quickly. These are your lifeline in the game. You lose, one, you are screwed. Defend them with your life (literally, it's better to lose your mech and start the mission over than lose an MFB and succeed).
All in all, MechWarrior 3 is an amazingly detailed game, that runs surprisingly easily and well for something so old. Though I haven't played it much, it is addicting, fun, and just enough of a challenge to get you to push yourself without really being frustrated.