I'd have to disagree with David, strongly so.
But then it is probably a matter of what "tag page" means for you and what you do with your tag pages!
Is a tag just something you slap on articles and stuff to simply have all a page that merely displays them all? Well yeah, then I'd tend towards noindex. I'd also wonder how many people really use a tagging like that - and have a use for it.
Personally I favour the other approach where tag pages are made into a page that can stand alone, for example giving a general overview over a topic and then listing the tagged articles for more specialised content. (Yes, you need a certain minimum of articles for tags to be useful at all ... and too many of one tag is probably also a warning sign of over-specialisation.) Of course this is not possible with every website and organisation. I have found, though, that such "hub pages" work exceptionally well for more generic than long term searches.
TL;DR: The importance and treatment of tags and tag pages largely depends on how you define their role.
Nico