I don't understand it as well. Some other coins come, and a few weeks, months later, you don't hear anymore about it. How can you know what new coin is a good one to buy for example.
I look at at two things: long-term purpose, and unique features or use scenarios in comparison to Bitcoin. Most of the alt market is a giant bubble because the explosion happened so fast and so recently, but there are a few coins I think are long-term gold.