Concrete is not a problem

The usual solution is to do a "cut and fill" ... remove dirt from the high side and toss it in the low side. The uphill side of the bed acts as low retaining wall while the downhill side is above ground level.

http://www.garden-share.com/profiles/blogs/building-a-tiered-raised

http://eartheasy.com/blog/2010/04/how-to-build-a-raised-garden-bed-on-sloping-uneven-ground/

Concrete can, over time, release lime into the soil and so may require close watching so the soil pH does not get too high. While this mostly adversly affects plants that prefer acidic soils, Rhododendrons, Azaleas, etc. it can also adversly affect other plants that grow in soil more near neutral.