What's up with semi transparent teeth?

Enamel. Enamel is translucent and can vary slightly between people. There is little dentin near the incisal edges, making the edges look transparent.
transparent teeth. Semi transparent teeth can be normal if you are talking about the bottom edges of the front teeth, or it could be severe wear. It is very hard to answer this question without more information. Do you suffer from acid relux (gerd)? Do you have a past history of bulimia? If you are concerned go see your dentist.
Enamel. If you are referring to the edges of the front teeth being transparent, this is usually due to the natural transparency of enamel. Also, severely worn teeth can look transparent.