Reuse of XSL in SharePoint 2013
After the big push towards search-based rollup web parts in SharePoint 2013 (namely, the Content Search web part) and their associated display templates, there hasn't been a lot of noise regarding how "old fashioned" Content Queries work with regards to styling.
Identical Web Part Settings
As you can see above, apart from the global 2013 branding changes, the web part settings are identical. In particular, take a look at the Styles section - it's the same! So not only can you use your 2010 XSL styles, but you can carry over all your web part settings. Although we cannot take advantage of the cross-site data retrieval that the Content Search web part brings to the table, you can bring forward all (or most, let me know if there are differences I haven't noticed yet) your carefully designed content query web parts directly into 2013 from 2010 with minimal changes.