The pipe bombs
To me, this is the movie's weakest point. Why show them making pipe bombs they won't even use and don't even plan to use? That scene had only two purposes: A) to show how driven and methodical the guys were - but several other scenes make that clear, so it was redundant - and B) to make the movie controversial by showing a step-by-step instructional video on making pipe bombs. It was gratuitous and useless and I didn't like it.
Otherwise it's a GREAT movie. The two lead (non-)actors are perfect and the dialogue only has a couple of forced lines. It's a resounding success.