Suggestion: add keywords (not just for todos)

You can use the backlink feature to track pages like this.

@dagurasu: Seems to me this way of workign is already supported by the backlin feature.Any further comments on this request, or can it be closed?

Hmm.. You are suggesting that everything related to the topic BlueBirds should, rather than flag a [birds:blue} value, rather simply reference BlueBirds. Then I can just go to the BlueBirds page and look at Backlinks. Yes this does produce the same functionality. It doesn't organize visually as well WikidPad.. you can't see a neat tree view with all topics and their referring parents (and you probably wouldn't want to because some kind of complete reverse tree would probably be a mess.) I still like the Wikidpad implementation, but I do see your point, and in fact, you have the advantage of allowing the "values" to have a hierarchy, whereas Wikidpad could.. but doesn't.

Visually though for this, and for reasons of wasted white space, and reasons of links being able to belong to more than one place in the visual tree... I find wikidpads index bar on the left to be much more usable at least for my habits. I'm using both regularly on different systems for different reasons (having very little to do with OS issues).

@dagurasu: I won't deny there is still a case for zim to improve keyword or tag based behavior. Just challenging you to make clear what is exactly the killer feature. Sounds to me the side pane usage to sort based on tags (or keywords) is what you are really after here, is that correct?

Yep... and an easy way to add a tag while typing without needing to slow down to do it.
I just realized too a problem with backlinking for this. For backlinking to work.. the page must exist. You can make a reference to a new tag/keyword/page what-have-you, but until you click it and edit something, that page doesn't exist and so you can't click on it to find its backlinks. The obvious solution to this is to make links in the index (and search tool) to un-initialized pages.