missed this when it was first posted...
--What are those "I wish I would have known this when I started" moments? For good and bad.
The level of comfort associated with violence the average criminal has needs to be accepted and translated into your comfort level with violence being even more in line than that of the average criminal. Normalcy Bias, complacency and believing there are things people will not do because they are "good" should be completely removed from your subconscious. This is done through hardship and experience of getting punched in the face and FoF. Go Boxing for several months, Thai Boxing/sparring, BJJ, traditional wrestling, jump on any FoF/Sim gun work you can find and your badge will get you access to. Get punched in the face by people who are stronger, faster and more experience than you in violence and learn from it during these lessons in violence. Then understand that you will have to employ that violence unconsciously at times, this is why training is extremely important.
--What are the "This is what they don't tell you" moments?
Be ready to use deadly force at a moments notice on everyone and anyone, then be ready to articulate your as much as possible. This means you should know every facet of case law, every facet of use of force doctrine, not just educationally, but have a thorough understanding of how it is applicable in reality, then how to articulate it on paper and in front of people. Every single officer should have a J.D. because of the level of complexity which comes with use of force, especially deadly force. Report writing and testifying are just as important as pistol and other violent skillsets. If you are a BJJ blackbelt and can run the FAST in 2.0 flat in your duty rig but you are a blubbering fool who cannot put a string of words together properly on the stand, you should seek employment elsewhere.
--What were those moments that made you realize the honeymoon period was over?
There will be a hell week. It may come sooner than later in some places, later than sooner in others. It will be a week that will literally make you question the existence of God, your role in the world and how you want to live your life. Call it a come to Jesus moment, it is when you saw the worst possible combination of events imaginable (young children killed, your partner killed/seriously injured, etc) and in the same week someone tried to kill you and you had to use a substantial amount of force to survive. It happens to everyone, that perfect combination of events. Some recover quickly, some never do. When this happens, do not solve your problems with alcohol or medication. This is where proper mindset, and preparing to win the fight are important.
--What toll did it take on your marriage, kids, family, and friends?
You will quickly understand which friends are worth keeping in touch with and which friends are baggage you should have discarded long ago. Do not have criminal friends, party-animal friends, whore friends, you should have friends who believe in the same fundamental things you do and accept the fact that you may have to have no friends at all because they may want to do something and your family should always come first.
You will miss birthdays, holidays, anniversaries, family gatherings and functions which your kids want you to be at. Do yourself a favor and go out of your way to take days off for those events. If that means you do not get to go on vacation that year because you ate up all your time that year on family events. Do it. You will miss so much otherwise. The job will be here, even if you are not, you are not missing out on anything and your family should always come first - this is why you should train hard, it is why you do what you do.
A police spouse (or significant other) is someone who understands the constraints of the job and understands that they have a very serious role in that officer's life. They are emotional support structure which exists to supplement the negative bell curve that officer experiences at work. They should know this, that is a talk which should be had early on. It may mean sleeping alone at night, it may mean not having direct contact for days sometimes, it may mean that 3am phone call and rushing to the hospital. Prepare them, explain to them the possibilities and responsibilities. If they love you, they will accept it and do what is needed. They are reason you come home at the end of the shift.
--What are the barriers to entry for a Non-Mil?
None, same for MIL. Don't be a criminal, don't get DUI's, don't abuse your significant other, don't be an asshole.
--How likely is a Non-Mil to be able to join other groups within the department or skilled organization with so many experienced veterans coming available and seeking careers in Law Enforcement?
Everyone has equal ability to go into units/assignments. If you want to be EOD, go get EOD training. If you want to be SWAT, go get a ton of training in various things. Be physically fit. If you want to be a detective, work your ass off in patrol and clear a lot of investigations. Actions always speak louder than resumes.
--What do you recommend someone participate in at the local level to get an introduction to law enforcement or police work?
Go for ride-alongs with perspective departments. Find local citizen academies. If you are young enough, the police explorers program is fantastic. Volunteer at your local Fire Company and/or get EMS certified and ride a bus around for a while until you get a degree and then apply to your PD.