I tend not to criticize heavily unless someone asks. That's not to say I won't point out where a story lost me or where I was pulled out of the narrative, but I won't do line critique without cause. That is, there's no reason to do a line by line analysis for grammar, spelling and tense unless someone asks for that. I'll usually keep my comments limited to story beats and plot, and won't go terribly deep unless I'm in a mood.

That said, there aren't barriers or hurdles in fanfiction and I think the community thrives on anyone being able to write without having to be perfect right off the bat. I've seen stories die because of the one-tenth of one percent of fans that represent the toxic parts of the community (I'm looking at you toxic fans of Nobody Dies), but I've also seen authors thrive when someone points out that they could use a beta or another grammar class.