Why not do it all: upvote the post, add comments, upvote the comments already there -- all as seems good to you -- that is do all or any part of what can be done.
I'm against arbitrary restrictions on principle, and your proposal seems arbitrary to me, but I am willing to be educated, so please teach me why it is not.
I still feel strongly that introduction post should have some awesome bit of content added to them, though my thoughts on the matter have slightly shifted. If you are a famous person, for example, I now see the benefit bring in your crowd base. Hopefully, their followers will bring in ten fold the capital that their post earns, so they are bringing a lot to the community. That being said my issue isn't much of an issue anymore. I do enjoy content filled intros more than single paragraphs, but I now see the value in certain ones. Even if I don't value what some people have to say, I can still value their awesome efforts in growing our community.
One of the reasons that I have not done an introduction is because I don't have any awesome content to include, and one of the folk that got me here said that in terms of articles, posts, whatever, long is better than short while I don't have anything long to say.
My only advice on that would be to start with a picture and a steemit sign showing your support. Talk about the feelings Steemit give you. Authentic and genuine posts can go a long way. Writing about what you know is always a good start, and no one know you better than you.