>Were laryngeals proposed on the basis that only consonants like [x], [G]
etc. (as opposed to, e.g., schwa vowels) were likely to cause the observed
effects?
>The only effect I can think of right now that would probably have to be
caused by a consonant is aspiration (Greek aspirated perfect, etc.).
Other effects, including vowel lengthening, are not uncommon. Think of how
the /n/ in Latin *-ns- *-nf- lengthens the previous vowel.
The specific effects of the laryngeals are various, and show up slightly
differently in different languages. They include vowel length and
aspiration, but also accentuation, and various kinds of paradigm
irregularity.
Once their presence was established, the next discussion was to identify
what they were. Because of the way they behave, they appear to have been
continuants, that is to say, not stop consonants, and not true vowels. But
discussion continues...
Peter