Recently there was a thread about the LOTR series being left off of some lists of the decade’s greatest movies. I’m not going to link to that thread, partly because I’m too lazy to search for it and partly because it’s relevant only as a jumping off point for this one.
So I ask you, Dopers: what, in your opinion, makes a movie great?
Greatness. Greatness makes a movie great.
Here are some of my off-the-cuff criteria:
It should bear repeated viewing, allowing the viewer to discover more in it each time.
The plot should be interesting and original, without detriment to the above.
The characteristics of the characters should be integral to the plot.
Ideally, it should open my mind to things (ideas, ways of looking at the world, feelings, anything really) that I haven’t considered before. It should expand my worldview.
That’ll do for a start.
Roddy
The movie should work as a whole. There shouldn’t be any extraneous or missing elements.
I have no idea what that is supposed to mean. You haven’t defined what elements are extraneous or essential.
One of my favorite movies is the Kevin Kline vehicle Life as a House. Another is What Dreams May Come. I like them both pretty much equally and would not hesitate to call either of them great, but there are plenty of elements present in the one that are absent from the other. LaaH has no special effects; WDMC is busting with them. LaaH is frequently amusing; WDMC never is. Special effects & humor are certainly elements of the movie-maker’s art, but in neither case does the lack of said element reduce the quality of either movie.
I think the measure of true greatness is lasting influence. This can be influence on later movies (“Alien” created a very new vision of space travel as utilitarian and industrial that influenced many, many later films), or other arts, or even the world outside the arts. There have been a number of films, for example, that inspired or popularized political causes, or drives for social change.
In other words, I’d say that what makes a movie great isn’t what it is, but what it does - and it can take a long time to figure that out.
Everything in the movie should feel like it fits. Which I’ll admit is a nebulous quality. Especially because in a well-made movie all of the plotting should be invisible.
But you can see when something doesn’t fit. It’s two different characters who the audience keeps getting confused. It’s a plot element that gets mentioned early in the movie and then forgotten. It’s the long exposition speech. It’s the gratuitous sex scene. It’s the character who is there in the sequel because he was in the first movie but doesn’t have anything to do. It’s the special effects shot that serves no purpose except to show off the effect.
Boobs. Boobs and car chases.
Hot, half-nekkid men. Or all nekkid.
It should engage me from start to finish. Every scene needs to be a masterpiece.
It feels true and sticks with you.
If you could pinpoint what makes movies great, then they’d all be great.
Apart from those by Ed Wood and Uwe Boll, of course. 
I think this post illuminates how “greatness” is such a shifting and intangible thing, because the two movies you mention are not only “not great”, but “not even good”, in my opinion. So clearly, what matters to you and what matters to me are different.
For me, there isn’t a metric, because any rule will always have an exception. There are genuinely great movies that I’d be hard-pressed to watch again or often. Some great movies have bad dialogue, bad acting, or low-grade production values. But in each case, what they have (a unique vision, openness of emotion, originality of perspective) may more than make up for those faults. Some great movies are remakes or sequels, some are plotless, and some are all but forgotten by history.
A great movie can languish in obscurity and is not obliged to be a household name, so sustainability is nice but not essential either. Some can start poorly, or end poorly, or sag in the middle, and while that’s unfortunate, it doesn’t stop them from being great. Great doesn’t mean perfect, but it does mean having a set of qualities whose combination make it singularly special, memorable, or inventive. And that can vary from greatness to greatness.
So I guess my answer is…it depends. 
If a movie is great, it was probably made between the mid-30s and the late-40s.
Flying monkeys.
Add a few explosions, and pack your bags! You are ready for the next Hollywood screenplay.
All you need to remember is:
Every robot movie is really “Pinocchio”.
Every rags-to-riches story is really “Pygmalion”.
Every love story is really “Romeo and Juliet”.
You just know it when you see it. Himself and I just watched Fear and Loathing in Las Vegas for the first time, and I absolutely did not like it, but I’ll admit freely that it was a Great Movie. It’s like art or pornography - you know it when you see it.
and the undead, preferably with Kate Beckinsale and Milla Jovovich.
It’s quite simple actually: Writing.
Great films have great writing. They make you think, they say things in new and clever ways, speeches and dialogue have rhythm. Shakespeare was the master at it, and it’s why his work is still studied and performed today. Nobody remembered who played Romeo in 1742, but they still get chills whenever he comes near a balcony.
I love action films. The greatest action films and the worst action films have many similarities. What separates the two is always writing.
It should never insist on itself.