The states would always work in their own self interest, even if it meant harm to the nation as a whole
Much smaller states would have the same amount of say as the larger states in the government
That the central government would always favour larger states over smaller states when when disputes broke out
The states would have no way to stop the central government from imposing harsh taxes on them
These are just a few ideas. I'm sure there are some more, but this is all that comes to mind