Yeah, like others on here, I too have been thinking a lot about this the next day... and wondering whether I have even overrated it. In the light of day, it looks even cheaper and hollower, once you take a look at how cliched the love story is, and how poorly it changes through the gears in order to get to its hugely unearned finale.
Example: the argument dinner. As soon as the scene started, I immediately thought "They are going to argue here" and lo and behold... Wouldn't have been an issue if it had been about something interesting, but it is another tired conversation about selling out and what being an artist means. (And it is filmed in the blandest shot-reverse shot manner possible.) While at the same time, the movie goes to great pains to totally skip anything to do with the actual content of Mia's one-women show (partly because it's easier to present on screen Hollywood's version of selling out - big crowds! synthesizers! funny costumes! than it is to present earnest artistic impulses, good or bad), instead leaving it as a convenient way to (a) split them up (with the most hackneyed contrivance of all time... "You didn't know the photoshoot was tonight?"), (b) make us feel sorry for Mia because two unknown guys are overheard saying she sucked, though we are not presented any evidence whatsoever, which allows then for (3) the deus ex machina of the random casting call because of said play...
And then they skip 5 years because the filmmakers are worried that the big number at the end would lose some of its power if we actually knew who was responsible for the romance fizzling away, if we actually had a read on the characters as actual living people... instead we get to handwave that all away to celebrate what basically amounted to a chain of meet-cutes then a love story completely captured in a single montage somewhere in the middle there.
Add to that the fact that it actually stops being a musical for a while so it can lever through the contrived plot and, boy, it's a terribly written film.