As mentioned grill chages, decal font style (no TRANS AM decal above driver side grill headlights) and black tail lamp bezels (actually started around the first week of Jan 1978), interior changes were made as well. Door panel style changed for both standard trim and custom trim interiors. Custom interior seat style changed.
W72 engine for 1978- Pontiac changed the part number in 1978 for the camshaft used in T/A 6.6 engines, so it has been rumored for many years that a change in duration and timing characteristics was made as well. Recently, good friend and overall Pontiac performance expert Rocky Rotella, proved this rumor to be true. Comparison between all three cams indicates the 1978/79 version has a few more degrees of intake duration with more aggressive intake lobe lift and gross intake valve lift using 1.50:1 rocker arm ratio than the 1977 cams. It appears the changes in the camshaft profile would result in approx 10 hp increase over the 1977 W72 camshafts. 4-speed W72 cars cam with 3.42: rear gears. There were other minor changes, such as air cleaner, vacuum hose routing, etc. If you need that info in can go into depth.
Hope this helps,
John