you didn't show your input so i'll have to guess,

if the doc is

<x>
 <a>one<!-- here -->two</a>
 <a>three</a>
 <a>four</a>
</x>

and the current node is x then

element()/text() will select four text nodes with values "one" "two"
      "three" "four"
so
      <xsl:value-of select="element()/text()" separator=", "/>
will generate one text node with value
"one, two, three, four"

* will select three element nodes, each with name a and with string values
"onetwo" "three" "four" so

<xsl:value-of select="*" separator=", "/>
will generate one text node with string value
"onetwo", "three", "four"

> Now i wonder why the result of the first expression contains no
> separator while the other one does. Any explanations?
presumably it selected a sequence of length 1, but without seeing the
input it's hard to say.

Isn't it that adjacent text nodes are being merged before the
separator is applied?