A major recurring theme in my study of computer networking is overcoming barriers to deployment. It seems that just about every proposal must address this issue.
By design, the Internet is something that everyone can use. It is the network of networks. The primary objective which motivated the original design of the Internet was to tie all types of different networks together into one network. Since the Internet is designed for such universal use, it is a special challenge to deploy any sort of substantive change.
Any change which might break compatibility of systems currently communicating over the Internet faces a steep uphill battle, no matter how green the grass might be on the other side. It seems that improvements do come, but require some sort of non-disruptive deployment path.
Deployment tends to be the domain of engineers, rather than researchers. However, it seems to me that deployment is central to research in this field.