The old method of mip-mapping was done on a per-polygon level. The same mip-map would be used for an entire polygon, even when some pixels of that polygon were better suited to a different mip-map. Gone is the per polygon mip-mapping of the TNT2, in favor of per pixel mip-mapping. Per-pixel mip-mapping, as the name suggests, can calculate which mip-map to use on a per-pixel basis, which yields more accurate mip-mapping.
By color-coding different mip-map levels, per polygon mip-mapping and per pixel mip-mapping can be easily differentiated. Shown below are two screen captures from Quake III Arena Test with color coded mip-map levels taken from Kert's GeForce 256 In-Depth article.