I think I'll break down my remarks into sections.
1) Ioan Gruffudd. He does a commendable job of taking his character from youthful enthusiasm to middle-aged despair, then to hope again. His performance would've been better served by a less fractured narrative, though. The story kept jumping around in time, so we were never with Wilberforce long enough to really understand him.
2) All the other actors. I particularly liked Benedict Cumberbatch as Wilberforce's
3) The parliament scenes. The movie didn't focus enough on the political process, but what there was, I liked.
1) Too much story, not enough focus. This is a movie that doesn't really know what story it wants to tell. The abolition of the slave trade? The life of William Wilberforce? The role of evangelical Christianity in 18th/19th century reform movements?
1a) Biopic syndrome. Did we really need all the dark hints about Wilberforce's laudanum addiction, especially when nothing was done to tie this in to the rest of the story? Did we need to know that he founded an animal-protection society and opposed the drinking of gin?
1b) The love story. I read an interview with the director, Michael Apted, where he talked about needing to find a love story to hang the movie on. And . . . no, he really didn't. The story is big enough in itself--it's about the abolition of slavery, for fuck's sake. What we get, instead, is more fragmentation. The bits about Wlberforce's courtship and marriage are barely integrated into the rest of the story (at most, they serve as an excuse for flashbacks) and they create real problems of tone. Plus, there already was a built-in love story, and it's between Wilberforce and Pitt. (You knew I'd get to the homoeroticism eventually, right?) Their friendship is the movie's real emotional center, whenever Apted can find any time for it (unfortunately, it gets short shrift in favor of the much-less-interesting marriage stuff). Oddly enough, Apted himself seems to have recognized both the centrality and the erotic energy of the friendship (he describes it, in the interview, as "not explicitly sexual," which has got to be the most delicious non-denial I've ever read). There's even a completely random and profoundly homoerotic scene in which Wilberforce and Pitt chase each other across a lawn while in a state of (for the eighteenth century) quite startling undress. And the deathbed scene with Pitt is more emotional and tender than anything else in the film. So why did Apted go with the marriage storyline?--except, of course, that Heterosexuality Must Be Demonstrated. (Yes, the real-life Wilberforce was married, but that doesn't mean the movie had to focus so much on it, to the detriment of the rest of the story.)
2) Nameless, contextless characters. In a historical film, it really helps to be told who the hell people are. When Wilberforce and co. are going on about the political influence of Lord Tarleton, and the audience hasn't been told that Lord Tarleton = Ciaran Hinds' character, it's pretty damn confusing. Especially when all the men are wearing wigs and it's hard enough to tell them apart already.
3) That fucking song. I don't mind the song itself, really, except for the fact that it's a Christian hymn and I'm not a Christian. But I hate that the movie was called "Amazing Grace," I hate that the hymn was sung twice onscreen, alluded to constantly, and played again at the end. It felt like condescension on the one hand ("ignorant audience, we'll give you a reference you probably know so that you'll feel safe") and like proseletyzing on the other.
4) Which brings me to the proseletyzing. Yes, Wilberforce was a Christian, and his religious faith played a big role in his political views. But the film didn't keep it at that biographical level--it seemed to want to shove Christianity down our throats. At the very least, I'd have liked to see some acknowledgement of the fact that Christianity was used by the pro-slavery forces as well--they were perfectly capable of justifying the slave trade with Biblical references.
4a) Amazing Grace was funded, in part, by religious groups, and I think that Bristol Bay productions is associated with "Christian filmmaking." I nearly decided not to see the movie after I saw that one of its "partners" was Focus on the Family (although, to be fair, it's partnered with a lot of perfectly unobjectionable organizations as well, from the American Library Association to human rights groups). Besides its funders and so on, the film's being marketed to church groups, rather like "Passion of the Christ" was, and that disturbs me a bit. But then, if evangelical Christians start getting all excited about ending the modern-day slave trade, that would be a good thing. It's a genuinely important cause and it would perhaps distract the evangelicals from the mischief they're currently up to.
5) Tidying up of history. As rozk has pointed out, the real William Wilberforce was as opposed to trade unions as he was to slavery, and the "moral reform" group he was involved with was instrumental in closing down "molly houses" and otherwise persecuting gay men. The film instead portrays Wilberforce as something like a modern progressive, a believer in "the voice of the people," and those moments made me cringe.
So . . . it's worth seeing for the acting, basically. But there's no reason not to wait for the DVD.