#1 Sight Picture is significantly different. Low wing, closer to the ground, more reclined seat position, etc.
#2 Energy management. The plane is harder to slow down...dumping the flaps won't slow you down nearly as much as the 172. And all that has been said here before.
#3 While your floating (if it happens) be patient. We have a 5000 foot runway, while you may use up too much of the runway for a touch and go, that's fine. Just let her settle, be careful not to balloon or bounce!
#4 Porpoise. You may flare too high (or balloon) causing a big bounce and cause a porpoise. If you do, GO AROUND...Smoothly add throttle (otherwise she'll cough), don't climb yet, just level her out just over the ground, get some speed, begin your climb, put the gear up, and collect your self.
#5 Control inputs are smaller than a 172.
#6 As she settles release most of your crosswind input. If you don't the nose wheel will touch down at an angle and you'll veer off towards the edge of the runway. If that happens, release the correction and then gently correct, if you over correct you'll have a bad day.
#7 Retrac. Normal stuff about retrac, but the manual gear on the Mooney is intended to be raised around 80mph. So, don't dilly dally about putting them up as it takes more force at higher speeds. When you move the bar, move it firmly and smoothly in one motion from the verticle to the floor and push the sleeve back to lock. If you can slide your thumbnail in the floor block, or the block when it is "down and locked" it is NOT locked. Push the sleeve a bit and you should not be able to put your thumbnail in-between the sleeve and the block.
#8 Cloud Ahoy is a humbling tool. I recommend Cloud Ahoy. Look it up, you can export the GPS track logs from the Stratus/Foreflight into it and look at your airspeeds, track, etc. It's very helpful in finding out why something happened or seeing how consistent you are.
#9 Your co-owners love pilot stories, beer, and wine...but always after flying.
That's all that comes to mind, I only transitioned 50 hrs (6 months) ago from C172/182s. It took about 10hrs to get it nailed.