You move from time- and voltage/current-domain,
to frequency domain. You work with integrated (low
quality PITA) inductors. You care deeply about
"second order" device attributes like gate resistance.
You may be pushed onto substrates which bring
their own bundle of weirdness (like SOI kink, RTN)
which may not be modeled well or at all.
You get to learn many new analytical methods and
tools (if you're the first guy making use of a new
device / passive, you may get to do 3D EM simulations
to extract a prediction of the parasitics; periodic
steady state and harmonic balance analyses for
harmonics, intercept-points, etc. which can't be
simply left to AC small signal analysis, especially
when talking about switch-mode PAs).
A good jump to make, if you're young and like learning
new tools and techniques. A lot to absorb if you're
of a crustier vintage.