The rules vary from state to state and can be pretty complex. The main thing I remember from Trusts and Estates class was the movie Body Heat with Kathleen Turner. Seriously, the plot of the movie turns on the old Rule Against Perpetuities (the movie actually gets the rule wrong but it makes a great plot) so the author of the text book had a note at the end of one of the chapters discussing the movie. It was one of the only interesting things in the class and why I remember the movie to this day. Oh yeah, I highly recommend it, even now.
https://en.wikipedia.org/wiki/Body_Heat
Anyway, the only other thing I remember from that class are the problems with holographic wills. That means a will written by the deceased. The problem isn't that they are self-written, it is that if people write the will themselves they don't usually follow the requirements for witnessing the will, and without the proper witnessing the will isn't valid. Some states recognize hand written holographic wills because they are self-evidently created by the deceased, but some states won't recognize a typewritten holographic will. I have no idea what the witness requirements are in my state and when I do my will (my current one is 20 years old and embarrassingly out of date) I will have someone who specializes in wills do mine.
I just don't see how the risk is worth it. Real property has to go through probate, so if you have rental property, you'll want to handle the executor and other formalities with some care. Your investments can be easily converted into a taxable event for your heirs if you don't handle it correctly, but it's easy to keep them out of taxes if you know what you're doing. It's like changing spark plugs: it's not hard to do, but if you don't know enough to recognize the signs telling you it's time to change them, you'll end up with a professional working on your engine soon enough. It's a thousand bucks. Sure, you might guess and guess right and have your will go through perfectly with no adverse tax implications and everything. But you will be just guessing, and what if you guess wrong? I have a buddy who tried to save $500 by not calling a plumber and doing a quick and easy plumbing repair on his own. A couple of weekends later he ended up with about a $5,000 repair bill and more new parts in his bathroom than he cares to remember. Don't make the same mistake with your will.
The only other thing I remember from Wills Trusts and Estates is the importance of where you put copies of your will and why you don't want to hide the key to your safe deposit box. (It's a safe deposit box, not a safety deposit box) There was a story of an old bachelor rancher in Montana or Wyoming who had been successful but didn't have a close family. He died with a substantial estate but no one could find a will. Years of litigation went by with no one getting anything but aggravation. Finally he was declared intestate (meaning he died without a will, I remember that from Paper Chase, not WT&E) and the estate was closed out. This was doubly a mystery because he had a reputation for being very careful and businesslike, and no one could imagine him not having a will.
Years went by and the family who lived at the ranch he used to own were doing some fencing and dug some fence post holes behind the old milk house. The post hole digger clanged against something, and it turned out to be a single key sealed in an old mason jar. Nothing else, no note, not explanation, no nothing.
After a lot of sleuthing, the family determined that the key was an old-style safe deposit key and they were able to track down the bank that uses that type of key, and from there they were able to identify a branch of that bank, and eventually the safe deposit box that matched the key. They opened the safe deposit box and found a sheaf of paper. The top page said:
"You'll find my last will and testament in a safety deposit box at xyz Bank in Billings. The key to the box is in an old mason jar buried ten feet due west of the threshold of the door into the old milk house by the barn. When you dig up the key you'll be able to get my will."
The lessons of this story are: use a lawyer, it's money well spent, Kathleen Turner is so hot it doesn't matter what device on which the plot turns, and don't hide the key to the safe deposit box that contains the stuff you're trying to keep safe. It might be so safe no one ever finds it.