Looking at Youtube videos today. Videos of the explosions at the Boston Marathon. I did this to see if my first impressions were right, and to see what the media couldn't (or didn't) show.
1. These were fairly small devices.
2. The finish line bomb had to be a fairly low order explosive. The blast wavefront wasn't moving very fast.
I'd bet AMFO. 15 lbs or so. Or something similar. Maybe black powder, but I don't think so.(ETA: others disagree)
3. Amazingly, once people got over the shock, many ran TO the site and knew what to do and did it. Not everyone, but a significant percentage of them. Heroes, each of them. I am impressed by the nearly instant reaction of those folks. I bet that many lives were saved because of this. It restored (a little) my faith in my fellow citizens.