The issue is not really one of rules light vs rules heavy. I think we can all agree that we like rules to be simple vs complicated. You can often get through 99% of a D&D session without coming up against things that seem awkward.
Rules get complicated when you try to do one of two things. You are either trying to get more detailed, or your dealing with a "Special Case".
A good example of rules becoming complicated in an effort to gain more detail is the Harn combat mechanic. Before I attempt to get into it, keep in mind I played this system exactly once ad a recent D&D convention. The system is based on the following.
1) Players make d100 rolls vs thier skills.
2) You then find out whether the attacker succeeded or failed, and wether the defender succeeded.
3) Determine if the roll is a critical success or critical failure. Any roll that ends in 5 or 0 is a critical something.
4) Consult a table, which tells you if you what happens for each success / failure combination.
5) You can then determine if you hit or were blocked, and roll a d100 to get your hit location
6) You then roll damage , and see how much was absorbed by armor.
7) Finally, if a wound has been inflicted, you then note the severity of the wound (determined by the amount of damage).
The above is a simplified run down. You also have to take into account if the defender is blocking, ignoring you / unaware, or counter striking. You can choose to aim high or low. You can go for called shots. This is a very cumbersome procedure. However, the detail you get out of it is quite remarkable.
Now lets see how we do this in D&D:
1) You roll 1d20 to attack an opponent, and add your attack bonus.
2) You compare the result against the defenders AC. If you rolled higher then the AC, you then hit your opponent.
3) Determine if the hit was a critical.
4) Roll damage.
Attacking in D&D is only complicated when you try to figure out all of the relevant modifiers to a roll, but you often only need to do this once.
Now, where D&D does poorly is in special instances. I will list them here.
1) Figuring out the result of a Jump attempt.
2) Attacks of Opportunity can cause problems since the causes are not always clear.
3) Having skills use abilities that dont make sense all the time (Intimidate based on Cha vs Str makes it useless for Barbarian types).
4) Polymorph other being a 4th level spell capable of creating "Save or Die" situations (I polymorph the dragon into a gold fish).
5) No Saving Throw for Harm making it more powerful then some spells of a higher level.
6) The Rangers favored enemy progression means that you will either take a +5 bonus at 20th level againts creatures you do not fight any more (Orcs) or a +1 bonus at 1st level against a creature that can wipe out the part in one round (Giants and Dragons).
Some of the above are the results of poor design choices. Others are the results of choosing a mechanic to cover a situation that is either difficult or does not always make sense. If you took out or replaced the special cases in D&D, you would find that the system is not very complicated.
END COMMUNICATION
