Two house rules we use in my games:
1. Bipedal 'Mechs can torso twist one hex facing right or left.
2. Jumping 'Mechs only get the +1 defensive bonus when they've jumped during the current turn's movement phase. (we're also testing giving the jumping 'Mech a +1 penalty to their to-hit rolls)
And something I'd like to see--if there was a way to make it work within the rules:
Instead of all weapons being lobbed into one damage value, I'd like to have four weapons categories with individual damage values for each: Energy, Ballistic, Missile, Artillery. So instead of only one dice roll, you'd have multiple--one for each weapon category that your 'Mech carries. That way, even if your energy weapons miss, you still have a chance of hitting with something else rather than the current 'one chance to hit or miss' rule. There's probably a cascading effect on other rules with this (like how do you handle Overburn?), but I think that having separate weapons categories better reflects the flavor of Battletech and expands the options for pilots during campaign play so we can put character points into making our pilot a missile boat ace or a ballistic weapons ace, etc.