This is an interesting question that took me some time to think about. I just purchased my first piece of ground here in Northern WI 3 years ago after my kids finished college and I had a little more disposable income. I started my journey right away after reading everything I could get my hands on. I was also fortunate enough to bring in a Habitat Consultant.
With that said here are my "Major" misses:
1.Not making the food plots large enough. In North Central Wisconsin I've got the only food plots for miles and attract deer up the ying yang. Each year I progressively make my plots larger. Each of these plots is carved out of the woods which is a ton of chainsaw work. This year another acre will be added by carving around each plot.
2. Brought in a dozer for leveling. This was just plain dumb on my part
3. Jumping on the "oak" band wagon. Planted several dozen hybrids, tube, etc. The deer would of appreciated another orchard more than those oaks - just saying. We have Northern Red oak all over the place.
4. Keeping the food plot plantings simple and stop trying to over complicate, manicure, etc.
5. This is a big one and a big investment, but my LOCATION needs sufficient enclosed box blinds for those "must" hunt days. E.g. Our 9 day traditional gun hunt in wisconsin was 1 degree w/o windchill. I was fidgety as well as my guests that were hunting - really hurt us.
6. Understanding cover needs and my tamarack swamp. Still hurts me. This is gold mine location that not a ton of habitat recommendations/information exist on.
7. Lastly, screening my neighbors better. all of them are Non-hunters and not full time residents, but they're all a serious pain in the ass. I would've thought that I would've had more issues from hunters on the national land, but that couldn't be more far from the truth. Hunters I meet on the National land have all been very courteous, respectful and enjoy some good conversation. I'm sure I'll meet a jerk or two, but so far so good.