Besides a couple of technical changes to the basic shapefiles released in the new Tiger2008 version, a lot more data has been included in Tiger2008 when compared to Tiger2007.
The two most important tables with Tiger are the edges (line segments like roads, rivers, boundaries, etc.) and faces (polygons made from edges). Here's a comparison count between the two versions:
As you can see, there are over 6,000,000 new edges in Tiger2008. That's a huge increase and represents new roads mostly that weren't in previous versions of Tiger.