That's an extremely subjective question and even if it weren't great is an ill-defined term. So prepare for an annoyingly long diatribe, but hey you asked
If I were trying to run a movie blog rating system it would go something along the lines of:
Acting Quality + Story Quality + Technical Quality = Rating, and also Rating + Subject Matter + Personal Enjoyment = Personal Value to Me
Take one that people have mentioned above, Inception.
Acting: Hardy good, Cotillard, Levitt and DiCaprio decent, Page was distractingly bad as usual and everyone else was passable.(3 of 5)
Story: Big boost for being original, bogs down a bit at times, interesting but shallow philosophic underpinnings (4 of 5)
Technical: Pretty amazing visuals and effects, Sound work was top notch (5 of 5)
Rating = 12 of 15 = 80%
Subject Matter: I like Nolan's brand of creativity, didn't have much beneath the surface though (3 of 5)
Enjoyment: Didn't regret seeing it in IMAX opening night, but also have yet to feel the urge to see it again (3 of 5)
Personal Value = 18 of 25 or 72%, definitely above average, but certainly not great by any means
For me a "great" movie is something that has a high rating and has subject matter that appeals to my mind or heart AND I enjoy (or enjoy breaking into with friends and/or fellow viewers). There's lots of Best Picture movies that I'd say are highly rated, but few that I'd say were great. Also I can agree that the subject matter's relevance to me might have no impact on someone else or even detract from a movie's rating.
A little known Irish movie a few years ago called Calvary is something I would call a GREAT movie.
Acting: Top notch across the board, from both pedigreed actors and no names. Brendan Gleeson deserved more attention. (5 of 5)
Story: Intense, Not formulaic or cliche, funny, deep, and a well crafted plot that didn't flinch or give up the ending, (5 of 5)
Technical: Well shot, beautiful cinematography and scenery, nothing groundbreaking, but nothing that distracted in the least which I appreciated (4 of 5)
Rating = 14 of 15 or 93%, and I'd argue most people who did see it wouldn't rate it too much lower
However when we go to the second metric.
Subject Matter: I'm annoyingly Catholic, I train people at Catholic Parishes to prevent, recognize, and interrupt abuse situations, I lived IN a small Irish Community establishing and training Irish for youth programs and worked extremely close with both Irish Priests AND people who hated anything to do with the Catholic Church. Also I now work managing a community for the formerly Chronically Homeless here in Austin and I deal every day with profound brokenness both in my residents, the people who volunteer and work with us and myself. All this background connects me on a visceral level to this film and it's themes, and ideas. (5 of 5, and higher if I could)
Enjoyment: The in movie experience is a little raw (although deeply funny at times especially having my Irish experiences), however it's one of my favorite movies to show new friends and then right after have cut the bullshit let 's jump in the deep end conversations to digest the movie and talk about what it means to have shit going on in your life and how to help each other through that. (5 of 5)
Personal Value = 24 of 25 or 96%
For me that puts this movie at almost perfect and truly "great", but for a lot of people that second metric would depress the value a great deal bc they don't give a shit about any of the subject matter and would be at best depressed by the movie and they probably don't want to discuss it so even if it was well rated they wouldn't give it a "great" treatment.