Normally plugins are associated with content management systems which you haven't told us what your using. So I'm assuming that you just require a system that does what you want it to do.
Pligg Social Networking
The first social platform that comes to mind describing your requires is a digg type social platform called 'Pligg'
Posts with Tagged Data
When a user submits a story (Post) they can tag the post with tags of their choice, this then is filterable and by a TAG cloud or search. Pligg is fully customizable so you can make the tags operate in the manner you would like, for example you can use an ISOTOPE type filter system if you wanted.
Up Voting and Down Voting
Pligg allows people to up vote or down vote, they can be anonymous or registered which you can configure in the administration panel. You can also set it so if they receive X amount of down votes it burys the post and if it reaches say 100 down votes you can set it to SPAM so it's deleted. So this allows people to administrate the site for you.
No Character Limit
Pligg allows you to set the character limit so if you set 0, then they have no limit... Though be careful with no limits as this can cause load on the server if someone is trying to abuse your system say with a 1million character post :P
Posts can be Edited
Pligg allows administrators and moderators to edit the post and it allows the poster to edit their posts or after X mins turn of this feature, Pligg is open source so there are many plugins that can be downloaded and written should you need more functionality.
Pligg out of the box doesn't look very pretty though you can download templates for pligg and even make your own, for example I made a simple design for Pligg Called clean Sam and it only took an hour or so and I released it on their website. But there are responsive templates and as I said you could style it anyway you want since the system is just like WordPress, Joomla and so forth and making changes is pretty straight forward.
Pligg is a Linking Book Marking Site.... BUT...
While Pligg is like Digg which is for social book marking sites this element can be turned off within editing the template itself. Just remove the URL feature and it'll be more of just posts and stuff like articles etc.
Don't Fall Victim of Pligg Spam
Pligg attracts lots of spammers, my first Pligg site that I made went to Alexa Ranking top 25,000 most visited sites on the planet and 99.9% of these were spammers using various automated posting methods to spam links. If you do decide to give Pligg a go ensure that you rename the register.php file and that you remove various other footprints they can use. This is noted on the Pligg Blog.
Don't Write off the Way the Default Pligg Looks
It's very easy to take a look at Pligg and write off as not being for you because frankly the default design is ugly.. Step out the box for a moment and if you can imagine a design then you can make that template or simply find one premade which is similar.