Oh you son of a bitch, you really want my answer it seems.
*sigh* Fine. Let me tell you what I think a good movie is using, in my opinion, one of the finest movies ever made: Martin Scorcese's Goodfellas. Keep a checklist with you, I'll be naming the points that, I think, make good movies good.
I'll start with the plot. Now, the film is loosely based off a true story, and it's an interesting story. It's about the rise and fall of mob associate Henry Hill, and the quarter century he spent working for the mob. His life takes twists and turns, and ends with him (spoiler for those who haven't seen it) in the Witness Protection Program.
Now, the plot may seem a bit simplistic, but it works. The best movies, in my opinion, have basic plots and make them interesting. Goodfellas' basic premise is twenty-five years in the mob through the point of view of three friends. Take any movie you think of, you can summarize it in a sentence or two. Pulp Fiction? Stories of criminals in Los Angeles. Birdman? Washed up actor known for superhero movies trying to make a successful play, but struggling. Shane? Drifter wanders into town and is shown kindness by a family, helps the town fight off the bad guy. All these movies, absolutely amazing, but when you think about, the plots are so simplistic, and that's why they work so fucking well.
Put that on your checklist. Has a plot that draws you in without having to do much; nothing stupid, but nothing complex.
Now, acting, that's key for any movie. If the acting isn't at least average, nothing will carry the movie unless you're looking to laugh at how abysmal the acting is. Goodfellas has some of the best actors attached to it. That's because they make the setting believable. They don't oversell, they don't undersell, they do it like it is. They make you really think you're watching mobsters do criminal shit.
Check: acting that isn't over or under dramatic. Makes it feel real.
Then there's directing. The director is, well, the director of the movie. They direct the actors on what to do, they direct the crew on how they want a certain scene shot, they direct the editors on how to edit the damn thing. The director is the glue that holds the movie together. Continuing with this glue analogy, bad directors are like Elmer's glue and good ones are like super glue. Bad directors, like Elmer's, manage to keep it together a tiny bit but not well enough. Good directors however, like superglue, keep that thing together tight.
Check: director is competent, tells people what they want done and exactly how they want it.
Cinematography is also important. Take Birdemic for example. The way scenes are shot makes it feel like a home movie, there's no interesting camera angles or anything, which makes it hard to watch. Goodfellas? Goodfellas has amazing tracking shots, has close-ups but not too much of them, and they manage to complement the scenes perfectly and keep the viewer wanting to watch even more.
Check: cinematography keeps the viewer watching.
Editing is another key thing. If a movie has bad editing, it suffers from pacing issues as scenes jump from one to the next. Goodfellas has great editing and great pacing. It is surprisingly short for a film covering three decades, being a little over two and a half hours long (contrast a trilogy with a similar premise, the Godfather trilogy, which is around twelve hours long). The movie keeps a steady, consistent pace throughout. No scenes are too slow, nor are they too fast.
Check: editing makes the movie a smooth, consistent experience.
Special effects, if you have them, are important as well. Goodfellas uses minimal special effects, so I'll point to a different movie for that: 2012. The movie is good for one reason, and one reason only: it has absolutely amazing special effects. It's a CG extravaganza, but they use CGI extremely well in it. Why? Because the scenes and shots are fast, not giving you enough time to focus and notice how it doesn't look real. Not to mention that the CGI sequences use almost exclusively CGI. Compare it to the recent Justice League. Superman's mouth looks weird because, when paired with a non-CGI background (and on an actual human) it sticks out like a sore thumb. CGI, if you're using it, must either be minimal or a fuckload.
On the other end, practical effects, I point you towards Labyrinth. The movie's use of practical effects is, like 2012 with CGI, amazing.
Final check: CGI is used in minimal amounts to avoid it looking like a cartoon, or in such a ludicrous amount in fast moving scenes that it manages to look real. Practical effects are good if you know how to do them.
BONUS: SOUNDTRACK
Ah, soundtracks. Goodfellas has an amazing soundtrack. The soundtrack must complement the movie's setting and scenes. You wouldn't watch Lord of the Rings if its soundtrack was all 80s glam rock, would you? Now, with Goodfellas, Martin Scorcese chose to go with music either from the year the scene was set in or earlier. So as the movie progresses from 1955 to 1980, you'll go from hearing swing to rock and roll. For example, the first song is Rags to Riches by Tony Bennett, a doo-wop song. What's the last song? My Way, by Sid Vicious, a punk rock take on the classic song.
Which brings me to my next point about music complementing the scenes. The opening scene of Goodfellas is, without a doubt, the best opening scene in any movie. It starts with three guys in a car, they hear something so they pull over to check it out, then open the trunk and find a man who's still alive. Two of the men kill him rather violently, then the third man, our main character Henry, closes up the trunk whilst narrating. "As far back as I could remember, I always wanted to be a gangster." He slams the trunk closed. Freeze frame on his face. Cue the music.
The movie draws you in with the mystery of what the sound that the car is making is. Then it shocks you with graphic violence. Then it draws you back in with Henry's narration, and the soothing sounds of Tony Bennett's voice. Not to mention, it fits the movie's theme, of the character starting out in rags and going to riches... Then he falls back down, which the song doesn't have, but still, my point stands.
And that's that then. Hope this fucking essay manages to give you a good grasp of what a good movie is, and how it's done. Also hope it was a good read.
~ Chris