A common question, difficult to answer without specific context, but at least one man has tried to answer it - hope the link is useful:

http://www.bbc.co.uk/worldservice/learningenglish/grammar/le...

what were they like? (very commonly used) did they talk alot, were they calm, did they seem happy?
This is what I would use if I wanted an overall impression.

how were they? this is not a common question - with respect to people.
Better options: How did they act? How loud were they? How well-behaved were the children?

I hope that helps

How were they?
To me, the asker is inquiring as to their health. It implies that he knows them from before.

What were they like?
Here, the asker does not know them and is asking about all those things you listed: appearance, demeanor, character, etc. Not about their health condition.

My thought from the USA.

#1 asks for a general description of a person: their habits, personality, etc.
#2 asks for a more specific, mood-related description.

Here's a possible situation for #1:
- What was Cindy like as a child?
- Oh, she was very sweet, cheerful and outgoing.

Situation for #2:
- I saw Stan last week.
- Really? How was he?
- He was fine. He asked me about you.

Hope this helps

What were they like? - I'm asking you to describe whatever 'they' were.

How were they? - I'm asking you to evaluate. Strictly speaking, I believe this question should apply only to a query about someone's health (physical, mental or emotional).

However, in common usage, it's just as likely to be asked about e.g. a new pair of shoes. 'How were the shoes?' meaning please evaluate the shoes. The reason I've added this is because I'm not sure what the 'they' is to which you refer in your question.

I'll look for some references.

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Note added at 19 mins (2007-08-29 20:21:36 GMT)
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"like1
• preposition 6 used to ask about someone’s or something’s characteristics. "
( From: http://tinyurl.com/2rnfoj )


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Note added at 20 mins (2007-08-29 20:22:56 GMT)
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"how1
• adverb 1 in what way or by what means. 2 in what condition or health."
( Also from: http://tinyurl.com/2rnfoj )


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Note added at 26 mins (2007-08-29 20:29:25 GMT)
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like PREPOSITION AND CONJUNCTION USES

2 If you talk about what something or someone is like, you are talking about their qualities or features.
What was Bulgaria like?...
What did she look like?...
What was it like growing up in Hillsborough?...
PREP
( From: Collins CoBuild Dictionary )


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Note added at 30 mins (2007-08-29 20:32:43 GMT)
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This, in fact, proves that the evaluation is not necessarily only with regard to health:

"how
4 You use how when you are asking someone whether something was successful or enjoyable.
How was your trip down to Orlando?...
I wonder how Sam got on with him.
QUEST

5 You use how to ask about someone's health or to find out someone's news.
Hi! How are you doing?...
How's Rosie?...
How's the job?...
QUEST
( From: Collins CoBuild Dictionary )

also see your question here:
http://www.proz.com/kudoz/2109671


It really all depends on context, especially for the question

How are you/were you?
How were you/they? mental/physical state or simply how did they feel?

Answer: they were okay but stressed out/
they were not happy

what were you like? more towards the character of a person or towards their behavior:

Answer: (shipmates lost at sea:) they displayed a strong attitude about it (did not give up) - they stuck it out
or: they were complaining, swearing (that's what they were like)