The acting is certainly a big part of the movie's success.
You can't expect something as unconventional as "Chasing Amy" (although this movie might be more consistent and hence better), which was a romantic comedy set in a geek/nerd world, with such accurate reflection of the people I know and the life I live that it remains a favorite of mine in spite of its rough visual look and some bad acting, but "Zack and Miri Make a Porno" is so well-written and contains such genuine emotion that it overcomes all the hurdles which a romantic comedy faces. The movie is hardly unpredictable, but it's well-written enough to survive the familiar, conventional outcome. It's a romantic comedy centered around the making of a porn film to help the two leads get out of a financial crisis. Oh, it's absolutely filthy, of course, but there's real heart here, and where I personally think "Clerks II" failed at bringing vulgarity and romance together into a cohesive whole, "Zack and Miri Make a Porno" does exactly that, including in the film's key scene between Zack and Miri (which also features a "LOST" joke! Did Kevin Smith make this movie for me?!) The plot is pretty much what you should expect going in.
In spite of, as I said earlier, some forced and unfunny race and gay humor, "Zack and Miri Make a Porno" still features characters who feel like real people (something Smith has always been able to do) and some really inspired dialogue and even some well-executed sight gags this time around.
The screenplay is no disappointment either.
"Zack and Miri Make a Porno" is a genuinely well-shot film, there's none of the awkwardness and amateurishness of some of Smith's other movies, and if Smith himself says that he thinks of himself as 'not a real film-maker' then he should re-evaluate himself because he has really achieved something quite surprising here. On a SModcast episode recently Smith said that he and David Klein (his cinematographer) had really worked hard at making this look like a 'real movie' (perhaps not the exact words, but it was something like that). Throughout his career Smith has been criticized for his skills as director (or lack thereof), and surprisingly Smith has been one of his biggest critics. The most instantly noticeable thing about "Zack and Miri Make a Porno" is how good the film looks. I'm almost surprised by it, as I thought there was no way it could meet my expectations.
With all that covered, does "Zack and Miri Make a Porno" meet my lofty expectations? In spite of some forced and unfunny gay and race jokes, it certainly does. It also happens that the relationship between Zack and Miri, platonic roommates who have known each other for a long time, is an exact reflection of my own relationship with my roommate. Also influencing my opinion of the film is the fact that if you put Zack in an art-house theater instead of a coffee shop and showed him Robbe-Grillet movies he would basically be me. I have also always liked Smith as a writer, and still count "Clerks" as one of the most true-to-life, funniest, and most genuinely inspired screenplays ever written, and all his other movies despite being more flawed in my estimation have something or the other to recommend in them, "Chasing Amy" being his best outside "Clerks". In spite of my dislike of "Mallrats", "Jersey Girl" and even "Clerks II" (which didn't gel into a cohesive whole for me, although it featured several great individual scenes), I have always liked Kevin Smith as a person, based not only on his podcast and Q&A sessions, but on my one, admittedly short, personal meeting with him. I feel like I should let everyone reading this know of the bias I had in favor of this film going in.