The Cornhuskers eliminated all of their rivalry games. Nebraska was rivals with darn-near half of the Big 12 conference! Since Nebraska has joined the Big 10, they supposedly have rivalries with Iowa, Wisconsin and Minnesota, but they are nothing like Oklahoma and Colorado were.
The second reason why Nebraska leaving the Big 12 was a mistake, much greater than the loss of rivalry games, is recruiting.
While Nebraska has historically done a decent job of attracting nationwide talent, when the Huskers left the Big 12, they lost their recruiting pipeline in the state of Texas. Turner Gill, Rex Burkhead, and Heisman trophy winner Mike Rozier all hailed from the Lone Star State.
There is even a website which creates an “All-Nebraska” team, using only players who were from Texas, and the roster is quite impressive. Nebraska's All-Time All-Texas Team
Nebraska does not play teams from Texas anymore, and a talented recruit from Houston knows that it will be much harder for his family to travel to a game in Happy Valley, Pennsylvania, than it would be for them to travel to Austin.