Parts to fill stock niches, including 0.625m LFO engines and tanks, a full suite of monopropellant engines ranging from 2.5m to sub-0.625m, brand new 0.625m SRBs, 0.625m high efficiency engines using LiquidFuel, Monopropellant or Xenon, a number of new probe cores with varied specialisations and shapes, more ways to produce electricity, a range of RCS blocks both large and small, and structural parts to glue it all together with.
To see what parts this pack contains, check out this imgur album.
Please see the the thread on the official forums for more information and discussion, and to leave feedback (I'm more likely to see it there).