A highly interesting question here, which is why, I guess, so many people have been debating the 'answer' for so long! Different methodologies and approaches to langauge teaching are based on different principles, so I don't suppose you will ever find one definitive answer to your question. Most methodologies and approaches are based on theories, e.g. of language, and of language aquisition - thus each methodologies will name different key principles of language teaching. Personally, I think it's important for a 'good' teacher ( I know, that's also subjective!) to have a general awareness of the different approaches to langauge aqcuisition and language teaching, and become and expert i when to apply which ideas. I do feel that in ecclecticism is one of the key "principles" for langauge teachers.

I agree to a point. 

I think we need a relatively eclectic approach to things like choice of lesson schemas/staging, activity and task types, genres & text types, particular sub-skills, how the board is used, whether cuisinaire rods are utilised*, etc, which allows us to cater to Ss' needs (and, to a reasonable degree, their wants).  That is, the practical aspects of teaching - the methods**, and the techniques**.

But I also think the reason we've started to say 'principled eclecticism' is because we're acknowledging that our choices are based on principles and theories of how people learn and how people acquire language.  And I don't think we have an eclectic approach to these aspects - the theories, or the underlying approach**.

And although there are still a lot of areas that need more research and some areas of contention, I think a large number of theories are generally agreed upon and supported by recent research.  

So ultimately, I don't think our principles or approaches can or should be eclectic.

* I never have nor will use cuisenaire rods myself, but I've met a couple of teachers who swear by them.

** I think the following distinct use of the three terms techniques, methods, and approaches is a very useful one for teachers to talk about distinct aspects of their teaching and beliefs:

• Techniques for tool-box-like techniques (eg. finger highlighting; use of timelines) and specific activities, etc.
• Methods for a collection of recommended lesson schema's and techniques.
• Approaches for underlying beliefs or theories of learning.

Hi! 

The answer to your question is that there are three principles, and each on studies some aspects that give to us twelve principles in the end.

The cognitive principle (related mainly to mental and intellectual functions)

- Automaticity

- Meaningful learning

- The antecipation of reward

- The instrinsic motivation principle

- Srategic investment

The affective principle (related to the emotional processing of human beings).

- Language ego

- Self confidence

- Risk taking

- The language culture connection

Linguistic principle (related  to language itself)

- The native language effect

- Interlanguage

- Communicative competence

 If you want to know each one read: 

Teaching by Principles: An interactive approach to language pedagogy

H. Douglas Brown, San Francisco State University, Prentice Hall Regents.

I hope I've helped.

Teacher from Brazil.