I was exploring an SE community I'm not a part of and got this message pressing the downvote button (on a not-answer) without thinking:
The first issue I have with it is repetition. It says Setting up an account, then Set up your account today in the next sentence.
Also, it gives a weird vibe of free to get account, but pay to get features (reputation), like several (not) free sites. SE is a free site and shouldn't have that vibe (it was also complained about before in the new users intro, but I can't find the link).
My suggestion is to replace the popup message with a small blue box (like the "You must have XX reputation to _____" message when you are signed in) under the button that says "You must be signed in and have XX reputation to _____".