The ambiguous nature of some of the plot points, and lack of explaining everything is rather nice. It harkens back to the OT in the best way: that feeling of you just dropping into a universe that has existed before you, and will exist after you.
Part of the fun of watching the original OT is not knowing the particulars, and engaging your gray matter to come up with explanations for the weird, exotic, rugged universe you're only seeing a small snippet of. One of the issues with the prequels is not just tha it tried to explain EVERYTHING, but it tried to give us a galaxy wide view of a conflict that should have stayed laser focused on two characters: Obi-Wan Kenobi, and Anakin Skywalker (listed in order of primary importance from episode 1-3).
Besides, from a business standpoint, Disney is going to be mining that 30 year gap for years and years to come. Toys, movies, video games, comic books, cartoons, and probably live action tv series. They didn't drop billions for Lucasfilm for nothing. They expect to make that money back tenfold.
Leaving questions unanswered in this first installment is good business, and, to the benefit of the viewer, good storytelling. Not every business decision has to be onesided in favor of the huge corporation. Sometimes the consumers get something too.
On another note. Anyone expecting this movie to be cinematic perfection is just inviting disappointment. Even the best films have issues. Questionable dialogue, plot contrivances, resolutions, acting, etc. films are huge productions, a LOT of hands go into the pot when you're creating a film (or a video game). Even developers and filmmakers who have cultivated a well oiled machine of a production process will still fall to the weaknesses of being a human being.
I'll be satisfied if the movie is entertaining. If I leave the theater feeling like I felt after seeing Fury Road or Creed, shit, that'd be great. It wasn't until later reflection that I considered the film's to be my two faves of the year so far. At the time I left the theater, I was just glowing from the experience of having seen a movie that delighted me, and triggered various emotions in me while watching it. I'll probably give a kneejerk impressions post of the movie when I see it Sunday, but I won't write my review until I've sat on it for a few days. I still have to write my Creed and Fury Road reviews, actually (REALLY behind on Fury Road).