The only way I know to be constructively critical is when I really like a song and want to sing it, but feel I need to change some bits, because that's the way I feel comfortable singing it.
I can't agree with mg about "It has to rhyme (well, not really has to..) exactly and perfectly, about 98% of the time. Every now and then you can misrhyme a little bit." I think for example using assonance instead of rhyme can be fine. It's not a second best, it's an alternative with just as much tradition behind it. And there are some great songs which don't rely on either rhyme or assonance. For example Woody Guthrie's Deportees (Unless you count Maria and Rosalita as part rhymes.)
And again with extra syllables, if you need them - there's no law saying that every verse has to have exactly the same tune, and in fact in much traditional music it's not even customary.
The crucial thing is, does it sound right.