Like above post said. When flouro stretches it doesn't bungee back. It stays stretched.and probably weaker. I've been told f.c. has a molecular structure more similar to glass than a nylon monofilament. Probably why it doesn't do well with bad kinks.
These comments from Clint and others are correct. There are two different things going on to differentiate mono and fluoro from each other: elasticity
. Elasticity refers to an object that can be placed under a load where it elongates but then snaps back, more or less, to its original size and length. Example: a rubber band. Plasticity is the ability or property of being able to be elongated but not the ability to then return to its original length and shape. Example: a stick of juicy fruit chewing gum.
Mono is more elastic; fluoro is about equally plastic but not as elastic.
And, fluoro does NOT burn from tying knots if it is "dry" and it actually has a lower thermal conductivity than mono. What hampers the knot tying is fluoro is just less "bendable." So, you can slobber on the knot as it is being formed but what it actually does is help it pull down tighter by reducing friction hampering a snug knot. Tying it dry doesn't "burn" the line any more, actually it'd be less, than mono would. It is just a harder material to snug up.
I like fluoro for leaders; I use it on one reel as a mainline and I struggle with it compared to mono or braid. But, many guys are so adept at casting that it is no issue for them at all.
Good stuff: yes, it is much less visible in clear water, it is much heavier so it sinks faster, it is not affected by freezing (ice fishermen use it for this reason), sunlight and temperature, etc. It lasts longer than mono which degrades pretty fast. And, once it has been stretched it is better for sensing a bite or nibble.