It's easy to see the negative things that occur in any environment. After all, no one is currently running a count of how many positive posts there were versus how many negative. We all have just a feeling about how things are going.
Regardless, SermonIndex is not OUR site. It belongs to the creators, who can do whatever they want with it. If they wanna lock a thread because you spelled a word wrong, oh well. That is there perogotive. But I think we all need to eat a slice of humble pie and recognize that they have let MANY threads go for a very, very long period of time, even after the discussion has devolved into utter nonsense. It is up to their discretion. We would do well to consider this.
Also, there are many great things that get posted on SI. There are interesting thoughts, there are ideas with which we don't agree, and there are people who exude godliness in the way in which they respond to things. Of course we all do not do this all the time.
If you want SI to get "better," then my suggestion would be this: learn to disagree without being hateful. We are not all going to agree. Even what we consider "encouraging" we don't all agree is theologically sound. The issue is that we simply need to understand that the very nature of this website, the very nature of the forums, and the very nature of humans (even saved ones) is to not all be perfectly united. Shoot, there are preachers on this site that contradict one another. Focus on the Scripture, say what is necessary for edification, and do it for the sake of the Church.
Lastly, Legalism is a very specific theological term. I know of very few on SI, if any, who genuinely believe that keeping the old testament law is necessary for sanctification or salvation. The moderators, by the very nature of who they are, what they do, and the fact that they have authority on this site are more than allowed to create, dissolve, or even on a whim enforce and/or alter any rules that they see fit. That isn't legalism. That's called being in charge.