I'm a bit late, but I find there are a number of extant issues with the design. Overall the functionality is good but it's not as intuitive as it could be.
- The check boxes aren't consistent with the OOjs UI, which makes the beta features page look a bit weird. I think it would be better to use OOjs elements, perhaps slider buttons like those used in VisualEditor, to differentiate from the already-used OOjs check boxes.
- New users could be asked to choose whether they want their settings to apply across all WMF wikis.
- It should be possible – particularly for the language option – for the user to choose to use the default options across all wikis, since some things have different defaults on different wikis.