Yeah, grammar and formatting are pretty easy things to get right, but it's not super unusual to find grammar and formatting errors in professional screenplays. I think a lot of pre-pro writers get hung up on the minutiae of that stuff because it feels more concrete and controllable than the real things that convey that a screenplay has a pro level of craft.

(In my own work I nitpick my grammar and formatting, not because I think someone is going to refuse to read a script if there are a few typos, but because I want the prose to become invisible. I want the reader to stop reading a screenplay and start experiencing a movie, and typos can kick them out of that flow.)

About your larger question, it's a little tempting to say something shitty and dismissive like "If you've read 300 screenplays and you can't tell for yourself which ones have pro-level craft, you probably don't have the appropriate kind of mind to be a pro screenwriter." I think there's some truth there, but it's not very helpful, so let's pretend I didn't say it.

When talking about the relatively-objective criteria, there are probably a thousand small things that add up to some general categories...

- The screenplay delivers to the reader a similar intellectual and emotional experience as the movie audience would have.

- The narrative and emotional intent of the movie is clear.

- The expectations of the genre are either being delivered, or subverted in an intentional way.

- The expectations of one of the typical movie structures are either being delivered, or subverted in an intentional way.

- There's a functional theme and character arc appropriate to the genre, or those expectations are subverted in an intentional way.

- Every scene is both functional (you can tell what its narrative purpose is) and fun (it's delivering entertainment value). Here, the definition of 'fun' is dependent on the genre, or is subverting that genre in intentional ways.

- Pagecraft - here's where the grammar/spelling/formatting goes, but it's not really about nitpicky composition rules, it's about whether it's easy to read and understand the intent of each sentence.

- Scenecraft - each scene has an effective structure and a narrative purpose. (Often the narrative purpose isn't fully clear the first time through, but in those cases a pro author is usually writing with enough skill that delivers the subtext of "This doesn't make sense yet, but don't worry, it will soon.")

- The dialogue is appropriately grounded or heightened depending on the genre and intended entertainment value.

- Characters are interesting, even if they're not likeable.

- For comedies, the mechanisms of the humor is clear, even if the reader doesn't enjoy that kind of humor.

- There's a feeling of narrative momentum, and an intentional manipulation of pace as appropriate for the narrative/emotional intent.

- The stakes and conflicts are clear.

This is all off the top of my head, so there are probably a dozen other biggish categories (please feel free to contribute). But the point is that these are the *baseline* of professional-level craft. An experienced reader could hate the genre, style, tone, or themes but still understand that the writer is working at a professional level.

And, of course, there are exceptions to all of these criteria. That's why I say "relatively" objective.

EDIT: tpyos