Alejandro,
I need you to clarify a little bit on what you mean by “without a Web Project”. For example, if you create a new Java Project, then create some JSP pages in it, and associate the JSP pages with the MyEclipse JSP Editor, you do get autocomplete for standard JSP tags but not with taglibs (this is straight forward as you don’t have a web application (and needed jars setup correctly) for the editor to complete against. Are you seeing this functionality with your setup? Were you asking about something else? (I’m using Eclipse 2.1.2, ME 2.7RC2 Linux)
As far as the real-time Syntax checking goes, that would be great but its really a matter of resources. If you want to vote for this feature I’m sure the ME team and others on the forums could provide their feedback w.r.t. how important or how much they would like this feature implemented.