The comments to this question answer it fully, but I think we should get a answer on record rather than closing the question as a "simple error" or "easily found in documentation" as neither applies.

Plot styling was [modified] in V10 with plot themes [and in other ways]. There have been several changes in the default behavior. [This is not] a bug, just a side effect of an intentional change.

The PlotRangePadding option can be used to restore the V9 behavior.

PlotRangePadding -> 0

or

PlotRangePadding -> None

will work.