You're correct - the current search only searches the titles. This is one of the most common complaints, and one that we would love to help reddit fix. One challenge is there are 5 to 10 times as many comments as there are threads. So a search index for that would be much larger, require more machines and RAM, and in the end it would be quite a bit more expensive for reddit to offer.
Oh, I understand that. The "search" feature of reddit is just so underwhelming that it doesn't even really feel like a feature. It feels like it's just a temporary kludge.
External sites are so much more useful it's crazy. Google lets me search comments and I can search in subreddits using "site:reddit.com/r/subname foo". Tineye + Karmadecay lets me even search for threads about an image or gif.
