This is something that I've been thinking about recently and commented somewhere else a week or 2 back as well-
In my opinion one of the better ways to leverage the forum for promotion is by making threads like this that either announce a new series or are maybe celebrating a milestone. HOWEVER what I see all the time are ones like you described where the OP is like "Yay I'm super excited and celebrating -insert whatever here-! Also everyone promote your stuff below too :D"
Honestly I think that just ruins the appeal of those sorts of threads? If the OP is taking the spotlight to celebrate something, I say own it and hog that spotlight! It's your thread and your promotion, you don't need to turn it into a link graveyard xD I like the ones that are focused just on the OP's series because that opens the floor for discussion about the series in question, how the series is going and how it reached that milestone, and if there is anything special associated with the milestone (i.e. a special illustration, or event, or etc.) in the comments.
Ones that invite people to hit and run their own links just become another generic promotion thread sadly.