It's been my experience with rules light that what is meant is "more abstraction". Whether this is accomplished well or poorly depends on implementation.
I agree. Games such as GURPS (and to a lesser extent Burning Wheel) are good because they have rules for almost any situation you'll find yourself in, but there reaches a point where the scale of actions requires abstraction to keep a game fun.
There is also the "Ooh, cool," factor. If I recall correctly, one of the BG issues with rules light is the game becoming a "May I please win now, Mr. GM?" situation. The games I've played (on both sides of the screen) that are rules light tend to be more focused on the players doing over the top, insane, ungodly, world-shattering acts and be opposed by the same. Nobilis pulls this off very well in my opinion. One couldn't design exact mechanics for an immortal power of every aspect of creation but one could design mechanics for an abstraction of relative power levels and keep it simple with: if you are more powerful you win. The trick for the players then becomes the resouce management of the points that let them temporarily go beyond their normal power level.
Now for a bit of Devil's Advocate:
Just as min/maxing helps develop a character a rules light system helps develop a gaming world and it's NPCs. Do such games lend themselves to an unplanned session? No, and the Devil would say that a game with more rules lets the GM come in with less prep. A duel of wits, a diplomacy check, a public speaking roll - they all allow the GM to fall back on rules and numbers if need be. In a system with little rules, the first to often come under the axe are those governing social interaction. The GM then must keep track of the nagging details as to what the NPCs want, what stereotypes would they project on the players, etc. The drug-dealer confronts a player moving in on his turf and what would convince him not to start a gang war?
Note that this all comes back to Lakira's comment on implementation. The players and GM both have to understand what is expected of each other and cannot fall back on the dice; some groups like this style of play and others don't.