...things will take at least 10 times longer to do than you expect
...some tradesmen are unreliable
...some tradesmen (not all) will try and rip you off or want to do more work than you wanted
...the good tradesmen take weeks/months to get a slot with
...when you start doing a little job you WILL uncover something that requires a lot of money and time to put right (case in point: I wanted a new light installed. The electrician came in for what I thought was a simple job, but then he found a load of really dodgy wiring that had melted, and found that our oven wasn't earthed. Cue a re-wire of the entire house).
...buying hardware is more difficult and time consuming than you think it will be. Think you're getting a radiator and all it's component parts? Think again sucker, you've got to buy all the valves separately.
...choosing decor can be a source of potential conflict between you and your other half.
...making a decision on paint colours can take months, and require you to paint a million samples on your wall.
...you will always run out of paint just as you are painting the last corner of a room.
...you will have to live with something that you hate for years. My artex palm tree in my hallway is still there after one year of us being here, but the hall is low on our list of priorities, so I expect to still be seeing it this time next year.
...your garden will look awful until you finish doing the inside of your house.
...the previous people who lived in your house will have been terrible at DIY. When removing two layers of wallpaper in our bathroom, I found that the previous people used filler to fill a hole in the wall, but didn't remove the wallpaper first. They then covered up their mess by wallpapering over the top. This is now impossible to remove. This is only one small item in a catalogue of disasters.
Anything you want to add to the list?