Why don't we use words as tags? Search engines already know that words are
tags. Why do we need to put an extra character (
#) in front of a word to make
it special? Here are two reasons I can think of.
The first reason is, that by using an extra character a website can make a link from the word that links to a search page or a special page for the tag. The problem is that if the post wasn't tagged then there would be no link. How can you find words that weren't tagged with the character? You would need an extra search interface for those words. But if you have that search interface you don't need to tag posts in the first place. Every word is a tag.
Second, the character also shows that the person writing the post actually meant for this word to be a tag. It's very specific. But there is no need to be this specific. As every word is a tag, you can still find a post if the words are written in there.
I think we don't need to use hashtags for tagging, just writing the words should be enough to help us find the posts we're looking for.