This bug has been fixed as of Mathematica 10.1.0.
The problem had to do with a conflict between the Suggestions Bar and the Dataset
paclet code. The same issue has affected other functionality using autoloading in a similar way, for example AssociationMap
as mentioned herehere or herehere.
It is very likely, as WReachWReach points out in a comment, that thisthis is a manifestation of the same bug.