Let's assume that the 'goodness' or 'badness' of teaching is objective (the opposing argument that it's relative gets lots of attention - and may be a bit pernicious!)
The objection to bad teaching fundamentally must be ethical: there are good and bad ways of treating people - waterboarding is bad for example (whatever George Bush thinks). Indeed the 'simulated drowning' induced by waterboarding has an analogue in bad teaching. As forms of suffering, how different is boredom or exam stress to drowning? (I blogged about the nature of boredom as suffering a while ago, see http://dailyimprovisation.blogspot.com/2009/10/tiredness.html)
But I don't want to exclude suffering from education completely: as an aspect of authentic human experience, it has a fundamental role to play. You can't learn to swim unless you have some concept of drowning. But there is a difference between the teacher who warns their students that "this stuff is really boring, but you have to work through it... it's worth it!" and one who continues on regardless and without acknowledging the element of suffering involved. What's the difference? It's between the teacher who reveals their own awareness of the knowledge, and of the learning process, and of their own experiences of learning and one who simply reveals a shallow aspect of the subject without any authentic engagement.
I wonder if authenticity is very important for good teaching: knowing something (the content) is linked to that thing which you know being 'real' to you. It ultimately is related to the 'purpose-form' of knowledge; it's ethical dimension; of being able to say honestly "I have experienced this to be good". It is to reveal your joy in something.
Inauthentic teaching, without the purpose, may be like the teacher saying "I'm telling you this because it's my job. I don't quite know how my job came to be doing something so meaningless, but there's nothing I can do about it, other than just do what I've been told to do"
What's interesting me now is that different aspects (subjects) of the curriculum lend themselves to different forms of knowledge, and the purpose-form may be more present with (say) inquiry-based learning, or life-coaching - where ethics and authenticity is high on the agenda, than (say) teaching chemistry - when content-form may play a bigger role (than in life-coaching), or computer programming where there's more scope for both the content-form of knowledge and the tool-form.
Maybe we can map the curriculum in this way??? Would that be useful?