1. There is no plot; there is only a series of incidents strung together into a feature film length movie.
2. There's no character arc. The people in this film remain the same shallow, irresponsible, unlikable- occasionally criminal- losers that they were at the start. No one is changed, improved, or matured by the events which occur throughout the movie, rendering it not only plotless, but pointless as well.
3. All of the main characters in Rent are despicable human beings. A movie is in trouble if literally every character in it is completely unlikable in every way, especially if the film has nothing else- like plot- to recommend it. Why would anyone want to voluntarily spend time with this bunch of selfish, self-absorbed idiots?
4. Every message that this movie espouses is morally and ethically reprehensible:
a. Family is unimportant. In Rent, family is consistently portrayed as embarrassing and backwards, while friendship is glorified as being the truly important connection in these characters' lives. Curiously, whenever family members are portrayed in the film, they are nothing but supportive- emotionally and, one suspects, financially. Yet Rent never misses an opportunity to sneer at them and make it seem like the protagonists are perfectly justified in despising them and jettisoning them from their lives.
b. People should only live for today, and for themselves. This is an attitude which permeates the movie and is spelled right out in the song "Another Day". In it, Mimi sings:
There is no past
I live this moment
As my last
There's only us
There's only this
Forget regret
Or life is yours to miss
No other road
No other way
No day but today
c. Everyone's life choices must be celebrated. As mentioned above, Roger is portrayed as being the bad guy for thinking that it's a bad thing that Mimi is a drug addled stripper. How dare he judge her chosen lifestyle instead of celebrating it. We see this idea repeated over and over again throughout Rent, especially emphasized in the character of Maureen. Maureen is an uncontrolled, unfaithful, insatiable nymphomaniac who appears willing to sexually accost anyone still conscious and breathing, and I'm not sure she'd be overly particular about that requirement. Of course, the movie portrays her as a wonderful free spirit who embraces- apparently literally- all of humanity. When Maureen's fiancee objects to her trying to seduce a waitress at their engagement party, guess who's considered to be in the wrong. That's right- Joanne is uptight and judgmental for expecting fidelity, and we get one of the major themes of the film at this point in the song Maureen sings, "Take Me Or Leave Me." It contains the refrain:
Who I was meant to be
And if you give a damn
Take me baby or leave me