We have one, but it's not fully enabled for ordinary users. (You can view and set keywords on nodes (via the Keyword Nodelet), but there's no easy way to utilize the keywords.)
tye and I talk about it in this relatively recent sub-thread. tye also said these important things about it.
Is it that little thing that says what type of node the currently viewed node is?
No, that's something else: node type. Each node can have only one, and it is determined by the system, and is immutable.
(Well, janitors can change a node's type, in some cases, but it's a radical change, analogous to re-blessing, vs. merely changing the value of an attribute.)
Between the mind which plans and the hands which build, there must be a mediator... and this mediator must be the heart.