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Isn't it annoying when you want to comment on an article, but don't want to go through the hassle of creating yet-another-user account at yet-another-website?
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We won't mind a bit.
Two alternate solutions:
4) Use the workflow module. Workflow, in some ways, is nothing more than a flat taxonomy that 1) triggers changes in the node when you change the flags and 2) allows gating to control who can change the flags and when.
You can create a 'votable' workflow, default it to no, and let administrators flip the votable switch on.
The downside is that currently I think there's only one workflow per node, but last I checked it was on the TODO list to allow multiple workflows per node.
5) Views Bookmarks
This is a simple flagging system, and is occasionally used as exactly that. It can be used to let users bookmark nodes for various purposes, but it can also be set 'global' and simply flag nodes on or off.