Theres been some poor decisions as far as design and choices in which nameplates would be offered. I think the biggest improvement with GM over the past 10 years is the engines, they are solid, powerful and fuel efficient. GM has the best line of engines in the industry.
Look at whats out there as far as 4s the ecotec is the BEST on the market hands down. I've owned 2 of them myself, they love boost and beg for more, the power they put out on stock internals is amazing. Obviously as far as V8s theres never been anything better than the LSx. As far as V6s the new direct injection engines are as good as anything on the market.
Now with all these great engines they have made very poor choices in which cars they offer. Things like the GTO, SSR, G5, G3, Aveo, Aztek, Malibu Maxx, these are simply cars that should have never ever been made. GM didn't do the wrong thing by getting rid of the Fbody, they were simply not strong sellers. Theres reasons for that, obviously we are in a time in which RWD is not practical for most people, people want cars that are more fuel efficient, you don't get that either with RWD.
GM wasn't willing to make the changes that Ford is willing to with the Mustang. Changes such as going to a turbo V6 engine as they will be with the Ecoboost engine and moving to smaller displacement V8s such as the new DOHC 5.0 engine. Gm could have easily made this change in the early 90s they already had an excellent platform in the Buick 3.8. They instead wanted to use cheap V8s that were less powerful and less mpg.
By clinging so hard to tradition they have doomed themselves into making any future traditions because they may not be in business.