What was the problem with the ending of TFA, exactly?
The fact that it doesn’t.
To explain: Certainly TFA has an ending, more of an ending than some movies at any rate. The problem is that it doesn’t have a Star Wars ending. In every Star Wars movie up to this one, we are able to significantly jump forward in time to the next installment, since the final scenes of each Star Wars movie before TFA have been some kind of statement or resolution (just put the words ‘deal with it’ after each of these summaries):
TPM: We saved the day. Anakin will be a Jedi.
AOTC: The Clone Wars have started. Anakin and Padme are married.
ROTS: The Empire won, our heroes are in hiding, and Anakin is Vader.
ANH: The Death Star blew up, and we’re celebrating.
ESB: We’re going to find Han no matter what.
ROTJ: The Empire lost.
TFA doesn’t end with a resolution or statement. It ends with Rey looking to Luke and basically asking him to train her. We don’t know what he will do.
And just in case you think I’m reading this as a question whereas it’s understood to be a resolution of some type, let me show you the end of the novelization:
Whether motivated by her stare or by something unknown, the figure finally turned toward her and pulled back his hood.
Luke Skywalker.
His hair and beard were white, and his countenance was haunted. He did not speak, nor did she.
Remembering, Rey reached into her pack and removed his lightsaber. Taking several steps forward, she held it out to him. An offer. A plea. The galaxy’s only hope.
She wondered what would happen next.
So it’s very clearly a cliffhanger and a question, rather than a conclusion and a statement.
Rian has even had to re-film this scene in TLJ just to answer this question. At the very least this is a complete departure from every other film in this series, and for no discernible reason other than that JJ simply had no idea where to take the story.