co- = subordinate vs. co = equals

From the Oxford Dict. Online:

usage: 1 In modern American English, the tendency increasingly is to write compound words beginning with co- without hyphenation, as in costar, cosignatory, and coproduce. British usage generally tends more often to show a preference for the older, hyphenated spelling, but even in Britain the trend seems to be in favor of less hyphenation than in the past. In both the US and the UK, for example, the spellings of coordinate and coed are encountered with or without hyphenation, but the more common choice for either word in either country is without the hyphen. 2 Co- with the hyphen is often used in compounds that are not yet standard ( co-golfer), or to prevent ambiguity ( co-driver—because codriver could be mistaken for cod river), or simply to avoid an awkward spelling ( co-own is clearly preferable to coown). There are also some relatively less common terms, such as co-respondent (in a divorce suit), where the hyphenated spelling distinguishes the word's meaning and pronunciation from that of the more common correspondent.
http://www.oxforddictionaries.com/definition/american_english/co-

Whenever I (a Brit) see the word coworker (we prefer to use colleague or workmate) I think it's something to do with cows. Joking apart, I've never heard of this supposed difference, and that Oxford usage note AnWulf quotes makes no mention of it. At Merriam-Webster, for the prefix co- the definitions they give are:

'1: with : together : joint : jointly

2: in or to the same degree

3a : one that is associated in an action with another : fellow : partner

3b : having a usually lesser share in duty or responsibility : alternate : deputy

4: of, relating to, or constituting the complement of an angle '

They make no mention of the use or not of the hyphen. I can find only a couple of references to this on the Internet, one being your own Linkedin page. One university site says - 'Do not hyphenate prefix except when forming nouns, adjectives and verbs that indicate occupation or status.e.g. co-author, codependent', but nothing about a semantic difference.

As far as usage guides are concerned, Merriam-Webster makes no mention of it that I can see. Fowler's 3rd edition (1996 British) gives simply lists specific words where the hyphen is used entirely for practical reasons, nothing about semantics, for example:

co-respondent (for the reason AnWulf mentioned)
co-pilot, co-signatory, co-worker ('hyphen used to avoid momentary perplexity of the reader')
co-op, co-opt (clashing vowels might lead to pronunciation as 'oo'
New words which could be confusing.

In Britain I think we tend to hyphenate most where co- is followed by a vowel, unless words have become very well established. Oxford gives both possibilities for cooperate, although it's so well known that there is no real need now for the hyphen.

And as for 'cochair', I have to confess that it looks like Gaelic to me (in British books, co-chair is far more common). I don't know where you have got the idea from that a co-chair was subordinate to the chair - that would be a vice-chair, wouldn't it? I've just checked with three American dictionaries and haven't found that definition in any of them. The Free Dictionary doesn't list co-chair, Dictionary.com takes you to cochair. This is from Webster's New World College Dictionary (at Your Dictionary):

"co-chair
noun
a person who chairs a committee, meeting, etc. jointly with another or others

transitive verb
to preside at or over as a co-chair"

But there are also admittedly examples where the co-chairs obviously assisted the chair, or, as in one 1959 example from the Music Operators of America (in Billboard), the Co-chairmen assisted the Chairman. And there's this example from the 2006 United States Code:

"The President shall designate a Chair from among the members. A Co-Chair of the President's Council of Advisors on Science and Technology who is not serving in the Federal Government and the Chair and Vice Chair of the President's Export Council shall serve as ex-officio members."

http://books.google.com/ngrams/graph?content=co-operate%3Aeng_us_2012%2Ccooperate%3Aeng_us_2012%2Cco-operate%3Aeng_gb_2012%2Ccooperate%3Aeng_gb_2012&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cco%20-%20operate%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Ccooperate%3Aeng_us_2012%3B%2Cc0%3B.t1%3B%2Cco%20-%20operate%3Aeng_gb_2012%3B%2Cc0%3B.t1%3B%2Ccooperate%3Aeng_gb_2012%3B%2Cc0

One that I find irksome is coleader(s) or even co-leader(s).
To me joint leader seems more natural but perhaps to some it may conjure up a somewhat different image.

I hadn't noticed the "subordinate" use before, but of course co-pilot is subordinate to the pilot. And in older TV shows, the credits began with a list of the stars, followed by a list of the (subordinate) co-stars.

Maybe we need to support the reëmergence of the diaeresis for clarity in pronunciation when we do not use the hyphen. We can reëducate the naïfs who have let it fall by the wayside. Too bad it only works with vowels...

@Hairy, maybe they meant "colander..."