The list of things I would do different is SO long... hard to say though because the things we do today - even the mistakes - make us into the person we are tomorrow... so maybe even our mistakes were a good thing in way.
Regardless...
#1 - Wouldn't have trusted certain people as much as I did...
#2 - Would have trusted certain people more than I did...
#3 - Would have taken more chances/opportunities that were offered that I turned down because of my silly professional "loyalty" issues...
#4 - Wouldn't have stopped building sites when I did and kept at least one foot in that door years ago...
#5 - Would have stuck up for myself more, been more assertive and not been such a passive, trusting, loyal schmuck...
That about covers it... at least the top 5... and most apply to the rest of the list as well in a nutshell... *sigh* Live and learn...