My perceptions of LinkedIn etiquette have evolved over time.  Here's my current view:

Standard for connecting.  When I first started using LinkedIn I viewed a connection as an endorsement.  I no longer see it that way, because I realized that my connections do not have direct access to each other - I have to approve introductions.  At this point, I am willing to connect with anyone with which I would normally exchange contact information (e.g., a business card).

Appropriate use of the name field.  Some folks insert additional information into the name field on their profile.  The most often additions are a title or an email address.  I understand that they do this because in some forms of search on the site the name field is the only thing that shows up.  However, I don't like it.  Not only does it seem aggressive in general, but also it screws up their contact information for everyone that uses the outlook toolbar. 

It's OK not to forward an intro:  People can request that you introduce them to someone else in your network.  While it's a bit awkward to say 'no', I think that it's appropriate to do so if it makes you uncomfortable.