Hashtags in Facebook are definitely going to be different in terms of usage, effectiveness and reach. #IMHO
I am already impressed by the cool highlight I can give my post with a hashtag, and also the fact that I can tag in my language too (which is Hindi) But there are few additional things I would like to see in Facebook Hashtags.

Ability to tag old posts

In the Facebook world, we've been surviving without hashtags till now, but now when they're here, I would like to tag my old posts too. That would enable me to link what I said previously to any of future posts by me or by others. This would be handy while using hashtags for sorting posts

Ability to tag a Post via Comments

Twitter has just retweets (RT with quote) for expressing your reaction on something someone else has said. But both Google+ and Facebook have comments too. Comments are important for a post. They might add something which makes the conversation on post vital for a topic or trend. Both Google+ and Facebook should allow tagging in comments, and include that post on that particular tag page - highlighting the comment.

Auto Tagging

Google+ auto tags my posts for tags it feels relevant, while I can delete those tags later if I want. These tags, along with the tags I've created appear in the side of the post. All this auto tagging and tag labelling is rather cool and very effective too. I might not know while creating a (specially public) post that my post contains a "taggable keyword" which can enhance it's reach.

Intelligent Popular Tag Suggestions

Similarly, suggestions while tagging would help me to use the exact words as others are using for popular trends. It would also help me find out what people are talking about apart from the top (say) ten trends. How cool it would be, if these suggestions could be intelligent - as in - they can give me additional tags or reorder tags for me based on what my friends are talking about and my interests.

Multiple Boxes for Trends

Off course the last, but not the least on this feature wish list would be Trends. Twitter and Google+ have it, and there is no reason why Facebook users should be left out from finding out popular trends. To be a little different, Facebook could try with categorical trends, or multiple boxes for simultaneously checking trends from various geographical areas.

So that was my wish list. Do you have one? Or you are a hash tag hater who thinks on the lines of, "Not on Facebook too!!" In any case, if you persevere till this point, why not drop a comment too?