1. For the second one, I think that its a good piece because its not overdone not crazy detailed just nice and cute. You understand it from the moment you look at it. Its nice and simple even with the colors.
2. For the first picture I thought this exemplified minimal but effective design because Its a heart everybody knows it as a symbol of love. Then, there are the little faces on either side. Boy and girl. At the bottom it says "We are not two we are one" and that just puts it all together. It also helps that the heart and words are the only things on there and that helps it stand out on its own.
3. For the third one I think that its good because even though its pretty complex on the bottom above the town you just have black and the moon. All the buildings together also make you think of a quaint little town. It only has two colors so its easy to look at. If ever building were technicolor you would have a hard tome looking at it. You're eyes wouldn't know where to settle.