Booting up gets hung up at around 50% I have a 2008-era 15-inch MacBook Pro, with Yosemite on it. The battery recently failed and, using only the power adapter, with the battery removed, the status bar gets stuck at 50%.

Is it only a battery issue, or is this a different problem?
Follow those instructions for single-user-mode, then when you are done, type "exit" and copy some of the first few lines of the output - on paper if you need to. I'm mentioning this because I skipped the exit step a few times, but more interesting things might come up there. When I brought the lines with me and showed them to the Apple tech (genius) he recognized them right away and knew what had to be done (clean OS install). Back up what you can and what you know is good; my hard drives were fine, but I didn't want whatever was causing the problem.