It's not really rules light. If you had to memorize all powers and the basic rule, your head would probably explode, just as with 3E.
The complexity has been shifted, though.
There is a number of base rules that everyone has to understand, like:
- Attack & Defense
- Save
- Action Types (Standard, Move, Minor, Free, Immediate (Interrupt or Reaction))
- Movement Types (Shift, Slide, Pull, Push, Teleport and simply moving)
- Skills
- Reading a Power Description
The fundamentals are not exactly hard, but they are already rich. Few games use such a richly detailed movement system, or the differentiation between attack, defense, saven and skills.
The real complexity is moved into the powers, and thus, they are kinda important on a "need-to-know" basis. If you have acccess to the power, you must understand it. If not, don't care. It is a bit like Fighter and Wizards - a Fighter doesn't really have to know how the Fireball Spell or Mordekainens Magnificent Mansion works.
The difference between 3E and 4E is that the complexity here is the same for everyone. Both Wizard and Fighter have powers - and thus a subset of rules - they have to keep track off and have to understand.