Nxt is light because of its Proof of Stake mechanism, and it is agile because the "protocol layer" consists of simple transaction verification, a blockchain mechanism, and a few core transaction types.

To use another analogy: if Bitcoin were an implementation of the OSI Model, its core might be seen as layers 1 through 4.  Nxt is layers 1 and 2, with upper layers left open for other people to build.  Nxt has stripped Bitcoin down to its best, brightest, beating heart and "reset" its foundation.  This allows for far more flexibility and agility than Bitcoin currently has, BUT leaves the advanced work (the "upper layers") to the community.