One of the things I really liked about the original show was that the Omnitrix was somewhat limited in what it could do. Ben would often decide upon what alien he would want to choose in a certain situation, but get something completely different and would have to be creative in how he took each alien's set of abilities in order to take on his challenges. Each Alien had a set of skills that differentiated them from the pack, and they all also had their set of weaknesses.
In Alien Force... Ben has mastered the Omnitrix. He can transform to any alien at will, and his collection of aliens he can transform into has expanded into aliens which mostly consists of two aliens from the original series spliced together. There are some new ones in there, but each alien has become so generalized that they all bleed together and start behaving the exact same. Instead of each alien being distinct, they have mostly boiled down to "Lazer shooty alien" and "Punchy kicky alien" with very little notable exceptions.
Not to mention, now that Ben has mastered the Omnitrix that battles aren't nearly as exciting or surprising for me anymore. I'll be honest and say that while I do enjoy character progression, Ben 10 wasn't exactly the best series for it in my humble opinion, so maybe I'm not the best judge of whether Alien Force was an improvement.
