I dunno... anecdata here, but my kids have quite a few sets - Batman, Ninja Turtles, Star Wars as well as some brick boxes and Lego City stuff - and they mixed them all together and now build whatever they fancy all the time. I was horrified actually, they don't care. In fact when I tried to build up a mini Millenium Falcon that's hidden in there I was told to clear off.

The new sets do have a few weird "only for that set" bricks, like batman helicopter blades or weird rubber Jedi hair pieces, but generally everything still fits together with everything else. I will say that there does seem to be a high percentage of small round 1x1 transparent studs. Every set seems to come with loads of 'em.