One example among many that have come up in play in my campaign: Teleporting. The text of the mental mutation "Teleport" reads,
"This ability allows the mutant to disappear from one location and appear in a location up to 20 miles distant without physically crossing the intervening space. If the mutant has never been to the location he is teleporting to or is only slightly familiar it (ML's discretion) he has a 25% chance of suffering 10d6 damage upon arrival as he teleports into a space occupied by another object, or misjudges some aspect of the location. If he is intimately familiar with his destination or has spent 2d4 hours conjuring an image of the location in his mind, he can teleport safely."Seems straight forward enough, right? This little paragraph has necessitated a whole host of rulings and house rules in play. Does only the mutant himself teleport, or can he transport his clothing and gear with him? If he can teleport with gear, how much weight can he teleport with? If he can carry fifty pounds of gear, what happens when he wraps his arms around the torso of a suit of powered armor worn by an opponent and attempts to teleport away with a section of the suit? Can he carry living matter with him? Does a location that the teleporter can see count as a location that he has been? Is there a practical limit on how many times this ability can be used in a day - in other words, could the mutant blink in and out of existence, traveling 20 miles at a pop, teleporting a mile or two up in the air and teleporting again as he begins to fall, and in that fashion cover thousands of miles in an hour? If the mutant has a mutation or ability gained from leveling up that grants extra attacks in a round, can he teleport and then make an attack in the same round?
While I have had no problem adjudicating these and other issues that have arisen with practically every mutation that my players have, I could see how some people might think that the game was incomplete because it does not explicitly address all the possible ramifications of the rules.
I guess this whole thing comes down to the role of the referee in a role playing game. If you are of the opinion that the rules are there to create a level playing field, to prevent the adversarial DM from taking advantage of the players, and likewise to spell out every action that it is possible for a player to take, then you are not going to like Mutant Future (or OD&D, or B/X, or Labyrinth Lord). If the group is okay with giving the referee the final say on rules interpretation and the referee has the trust of the players, then you may well find such rules lite games amazingly liberating. I have slogged through combats that took hours in 3e and I am currently playing in a 4e campaign - while I personally enjoy the tactical thinking that the 4e rules encourage in combat, I have to say that far more interesting things happen during combat in my Mutant Future campaign precisely because there are very few rules governing what the players can do.