The developing S7 object system (https://github.com/RConsortium/S7) is looking fairly promising in that it combines many of the nice properties of S3 and S4 (validation, multiple dispatch, sane constructors) while still being fairly simple and straightforward to use.

Excellent news. Quite promising, but R's power is its been actually functional natively. Even binary operations are functions, `+`(x,y) would work as in x+y