I think project team members and developers are the most important factors. Their knowledge, their background and other projects that they collaborate with.
They should have clear Roadmap that shows all the details they want to implement in the future and the estimate lunch time.
having an active community that you be able to ask your questions and have their support.
These factors are the most important one but, you should search about variety of things before investment
you are right, and also To invest in an altcoin, the most important thing is to investigate the project to know what future we can expect. A token that does not have an important project behind it will most likely end up in oblivion or worth pennies. That is why you have to investigate the project and see the future it has.