What are the difference between a crossbow broadhead and a regular broadhead in the same weight?

If you are talking fixed blades, nothing. Mechanicals will have a stiffer blade retention since some crossbows are so fast.

I can attest that there has been no difference that I have found with "fixed" broad heads that I have tried as part of my crossbos set-up. I have used the identical broadheads that I had excellent success my last few years of bow hunting, before I had to go to a crossbow. These all have done fine.

crossbow shafts are thicker as are the broadheads...at least that's what Ive seen...the threads are the same size

thanks for all the input guys!!