Now to be honest I know nothing about scripts BUT I do watch a LOT of films, TV etc. to know a good script when I see one. And I know what I DON'T like to see..
1) everyone doing everyone. As much as most TV show script writers seem to think this goes on in reality, it does not. Ok, granted, it's a small world, but it ain't THAT small..
2) drama where there is none. Now this is particularly frustrating. And it ruins most shows that go on further than two three seasons..
3) characters evolving into annoying. I hate when that happens, again concerning tv shows lasting over a certain period. This is when "you've lost that loving feeling" comes to mind.. and not in a good way. (is there a good way?) - happened to Buffy come 4th-5th season.. :/
4) bullshit. This is a hard one, because it varies depending the genre of the film/show. Like if it's a horror movie there are things that couldn't be possible and it's totally acceptable. But I hate when this happens to action movies. And yeah, I get it, people wanna see rad stunts and that's just dandy, but does everyone have to do the "flying kicking" trick? Srsly, the bullshit action totally ruined Charlie's Angels the movie. The show was all about badass chicks being all under cover and sexy and clever but they didn't need all those shit "she's hanging on a wire" moves. They had the real deal and it was SO much more entertaining..
5) when things don't make sense. The "why would he do that?" or "wtf?" parts of films/shows. There's times when it works - like in real confusing thriller/psycho flicks - but it mostly looks just like the writer lost his/her mojo and is trying to add fluff to his/her stuff..
6) this one's a TV show classic - especially in Czech shows and just let me tell you the outcome is so pleeeeeease - characters returning from being dead or when they were "away". Worst is when it's the characters you were happy to be rid of. It's so old and so tragic.
7) this one's a movie sequel ordeal - characters NOT returning. How annoying is it when your favorite character gets fucked off! (Appearently almost happened to Samantha in SATC - well bloody happy it didn't!! She is sooo my fave!)
8) when it's no surprise at all. Too much predictable is just a waste of time. I mean, common, can't ya come up with anything new??
9) when it's different than in the book. I hate it when films fuck with the literary original.. Actually, let me rephrase that, when they fuck with the book/short story/comix whatever in a bad way. I know this is a toughie with all things considered, but still, you have to admit, seeing your favorite masterpiece turned into a crap movie hurts..
So basically, the answer to how to write a good script lies in the omission of these trivial mistakes. Oh and keeping it real and close to the heart helps. If you believe in smth, changes are there's smth there to believe in..