Lower air consumption is the biggest spin off for scuba divers from freediving. It doesn't matter so much if your on a rebreather, but can make a huge difference when on open circut.
Another thing that goes with it is more efficent movement through the water. Since we can't waste a lot of energy when freediving we can carry this over into scuba for longer, more relaxing, dives.
One of the biggest things that would help tech divers, and others who are rigged DIR, and donate the primary from their mouth, are exhale statics. There's nothing like exhale statics to increase your comfort level of having to give away your reg after an exhale.
I no longer do the deep-air stuff, since I switched to mix, but there where lots of things that crossed over to that kind fo diving- like CO2 tolerence training, which helps when dealing with narcosis. Also, the facial immersion breathing on the surface before a deep air dive helped to slow everything down. It even helps to slow thing down for a recreational scuba dive, especially one that involved a long surface swim to get on site.
Another area is more precise weighting. Now that divers all use BC's they put on HUGE amounts of weights- rather than take the time to actually figure out how much they really need. Freedivers are very tuned in to exactly how much weight they need and are careful not to go over on that end- even a heavily weighted spearo won't have on half the weight that a newbie scuba diver wears.
Of course, there's also nothing quite like scuba diving with a pair of long blades on when it comes to diving in currents.
The monofin dosn't work as well for the scuba thing- been there, done that.
Freediving masks are also pretty nice for scuba diving, although I am not crazy about using my Sphera for scuba while it's my favorite mask for freediving. A nylon exterior freediving wetsuit also works out really well for scuba diving and I have made many a scuba dive in my freediving suits.
I am sure that there are others that we haven't talked about yet.