Your comments

I just thought of a great use for this one: grouping synonyms and similar stuff (without actual hierarchy):cat+cats, pics+photos+fotos+фото etc.
Or you could just color notes red if they are tagged "red", green if "green" etc :)
I'd love to be able to specify a template. E.g.

"%date: %title\%tags\\%text\"
"01.01.1900 16:04: My title
some, tags,

Text starts two line-breaks later.

And since a template is a string, it's easy to store/provide examples.
1) and/or drag-and-drop would be perfect.
... but limit the maximum width to N%?
Or allow every character except for the tag separator. Use "," instead of " ". Nope? :)
That explains it. Thanks.

My fear is that the tag-list will become more complex (and it's a drawback, I suppose). I can't imagine an uncluttered AND non-repetitive UI presentation.

For instance, how many times would "c++" (being part of "programming") appear in the list of tags? Would the list become a tree, or would the "c++" just be displayed twice?
(second appearance to somehow indicate that the tag is also a part of "programming")

This concept would also likely require additional dialogs/menu items to be introduced, since the tag would now have more than one property (name).

My solution would be to just tag the appropriate note with both - or all the needed - tags, and stick with the concept of saved searches as a separate, plain and simple layer of organization.
What you described is called "saved search". Please, do not corrupt the idea of tags. Use prefixes, or suggest a "saved search" feature. After all, hierarchy - and it's drawbacks - is the very reason why tags where introduced in the first place.
Voted for, of course. Optional - agreed, no toolbars - great. But no markdown, please. It's ugly and complicated. I would prefer the mark-up google talk uses (*bold*, _italic_) or the one used in 37signal's writeboard (the best option, IMHO).