Essentially the 'TweakSet' was an update to WD5. Once installed to the Designs gallery, they were as easy to use in your designs as any other built-in feature.

Tweaks are pretty much just pre-made shorcuts (helpers) to add certain additional Web page snippets to your site which you could add manually anyway. The snippets are added to a placeholder either by yourself or dragged from the designs gallery tweak set.

I'm sure that any tweak which has proven itself during the WD6 life-cycle would make it into WD7.