Gameplay
Gameplay is one of the most unique aspects of Mechwarrior 3. Yes, it's a 3D first person game, but this isn't Quake or Half Life. You're not going to be running around wildly in all directions because you're piloting a vehicle here. The two hardest aspects are first, the use of the throttle, and two, torso twisting. The throttle control is crucial, because it determines your walking speed, which in turn determines your turning radius, etc. It's not good to always be running, because you'll need to turn in tighter at times to keep your guns trained at smaller, faster mechs.

The view from a Bushwacker cockpit
Twist and Shout!
The torso twisting is probably what frustrates a lot of people the most. Since you can turn the upper part of the mech independently from the bottom, it is possible to point in one direction while running in another. You're given almost a 180 degree arc in which you can swing your torso. This gives larger mechs a chance to track down the smaller mechs that are trying to get behind them. However, you run the risk of running into walls or other obstacles if you don't master control. Veterans of MW2 and other earlier mech style games already know this stuff, but newcomers should be aware of them before they get frustrated too early and give up on the game.
Shooting - not just about pulling the trigger
There are other important aspects to mech piloting like shooting. One of the nicest new tricks is the zoom feature. In MW2, the zoom would zoom your whole view in. Now, holding down the right mouse button zooms your target reticle in a small window. This means you can get fine control of your aiming, while you retain overall vision of where you're going. Awesome! If you use a lot of laser weapons, you have to keep a sharp eye on your heat meter on the left. If you start to overheat from firing too quickly, the mech goes into automatic shut down mode. Shutting down the mech turns off the engine and all electrical systems, leaving you a sitting duck. Like in MW2, you can override the shutdown, but risk explosion from the excess heat. Add more heatsinks to get better heat dissipation.