Maybe I've simply been on the internet far too long (I remember when images were the newest, bleeding edge browser feature!), but when tagging came about on livejournal, I thought it was awesome. Then Twitter did the hashtags, and that was neat-o (especially when google picked it up with G+, too), and tumblr has its system (which I don't fully understand), and AO3 implemented tagging awesomely too--it's great to help you find the fics you're looking for. I love that you can nest tags on Dreamwidth (presumably lj has this too), and it's a really great way to organize and make accessible content for readers and users--and yourself.
Except now it seems like people have lost the plot and forgot what tags are actually
for.
They're for organizing and flagging data, so when people put commentary in their tags, or use their tags for a summary, or pretty much tag things with any single use tag they will never use again (and neither will anyone else), it's bad practice and unhelpful and it makes me a little bit cranky. A good tag has semantic content: it's descriptive and helpful and points the reader to the content it describes, and it has the potential to be used more than once.
I say all this knowing full well I'm terrible about tagging consistently, and I can get all kinds of dysfunctionally obsessive about it, but I do try.
ETA this is mostly in response to seeing tumblr style tagging on AO3.
Seriously though, I am way too prone to this:
( XKCD: Duty Calls )