I agree with what you said about the 13th Amendment. Perhaps some people thought it was going to be a film about Lincoln's entire life or his political career. Actually it's just covers the last several months of his life, with the focus being around the passage of the 13th Amendment. I also agree with you about Thaddeus Steven's relationship. It wasn't the climax of the film, but it added an emotional rationale for his strong beliefs in racial equality.
If you are interested in this film it is about politics, with a lot of speeches and deal making so prepare yourself for that, and if that isn't your thing you might want to skip it. The biggest action scene takes place at the beginning of the film.
The cast was very good. Daniel Day Lewis was great as Lincoln, and both Sally Field and Tommy Lee Jones shined.
In time, Lincoln will probably be regarded as one of the high marks of Spielberg's later career. Personally I have no desire to see it again, but that doesn't take away that it was a very well together movie.