What will happen if I don't specify a version for a Maven dependency?

I don't think you can do that in the ordinary dependency section of your poject. The only case I know that this is allowed is when you inherit a parent project with a <dependencyManagement> section, which is used to coordinate dependency versions across a set of projects. Then you can use the "shorthand" definition, without the version. But it would still have a version, which it inherits from the dependency defined in the parent project.

There are some other places in the pom you can drop the version as well, but those are not directly relevant to including dependencies (like plugins/exludes etc).