I'm not sure about you, but when I evaluate a film, I ask myself these questions:

1. Does the film succeed in telling a good story ("good" as in well structured, clear unless intentionally obfuscated, has a beginning, middle, end, and clear plot threads through each part?)
1a. Are the characters successfully drawn?
1b. Are the situations plausible?
1c. Does the plot flow from point to point logically and with some sense of ease?

2. Does the director's work in staging the film and the choices in acting, pacing, etc. enhance the film, hamper the film, or keep it at a median level of quality?

3. What is the quality of the acting?

4. What is the quality of the visuals: cinematography, production design, art direction, hair/makeup, costuming and, if applicable, the special effects?

5. What is the quality of the music or score, if present?

6. What is the quality of the sound mixing and sound design?

7. Do all of these disparate elements work as a whole to present a cohesive picture?

Here's Purple Rain:
1. No. The story is all over the place, and the high variance of the plot in earlier drafts bears witness to the fact that this movie was less about telling a story with Prince as the lead than it was about just promoting Prince.
1a. No. See above. Prince's clique is generally cast to play heightened versions of themselves, but this ends up resulting in either throwaway characterizations (Wendy and Lisa) or inconsistent characterizations (Morris Day).
1b. Uh, generally.
1c. Not quite. Film meanders all up and around the plot, which is essentially an A-story of boy-meets-girl, boy-loses-girl, boy-wins-girl back and a B-story that's a modernized variation on the mortgage on the farm/let's put on a show plot (see also, "Can't Stop the Music", which is sort of like "Purple Rain" for the Village People, but even worse)

2. The director is really there to make Prince look cool, everything else be damned to hell. He succeeds at that task, but the rest of the film suffers from inattention and a lack of focus that makes it feel long, long, long.

3. Oh, come now.

4. Visuals are decent, save for some awful location night-shots.

5. Oh, come now, but for the opposite reason: one of the best song scores ever.

6. No complaints about sound quality

7. Hell naw, they don't!