General
Because it provides the JavaEE Web Profile by default. If you need full JavaEE support, read the Usage Page.
By default, GlassFish 4 Web Profile is used. Supported versions are the Full and Web Profiles from GlassFish 3.1.2.2 and GlassFish 4.0.
Yes. This is one of the killer features when running a web application in an embedded Jetty or Tomcat container. JavaEE containers generally do not support this.
The Embedded GlassFish Web Plugin plugin is different in that is synchronizes any changes to web resourcres and classes. Your changes to web resources will be immediately visible, and when you use a solution like JRebel, so will changes to your classes. Only changes to libraries still require a restart.