Search itself isn't the magic of hashtags. The magic is that a hashtag name is actually commonly agreed unique string ID of a theme or trend. How many unrelated results would you get if you search just for Google?
If I want to talk about Google+, I'm more likely to use a hashtag, such as #googleplus, I've seen before. I could be a rebel and use #gplus, but it makes more sense to use already-established tags.
If we just experimented with hashtags on HN for a day or two, I'm sure they'd be hard to drop. (Although currently, I believe # is not allowed in titles)