I'm reminded somewhat of the discussion of "Quality" in Zen and the Art of Motorcycle Maintenance... we know it exists, you just can't quite put your finger on it.
I tend to agree with those that have suggested that "recreational" vs "technical" is not a rational split... recreational sits along side commercial, military, scientific etc. Recreational diving is when you dive for fun.
Limiting the following to when people are diving for fun!
Nearly every definition that you could put forward for "technical diving" is flawed... so you could suggest that it's dives where the perceived risk of the dive is such that it requires additional planning, equipment and so forth.... well, a new OW diver can have a pretty serious risk of running out of gas, so that should be technical diving, right?
You can suggest that technical diving is when you can't make a direct ascent to the surface... well, I recently did a dive in a lake that is used for water skiing so you couldn't make a direct ascent to the surface... so it's technical, right?
You could think that cave diving is technical diving, but does that make technical diving just cave diving? I don't think so.
Trying to define it is, IMHO, rather pointless. What I might consider a "technical dive" (say a 55m dive, for 25 minutes BT, using at least two gas mixes) is probably routine for many - so it probably wouldn't be "that" technical for them.
I'm fairly comfortable having an amorphous, indefinable term. It's very individual, anyway. But, for me, a dive is a technical dive if I am really thinking about who I'm doing it with and why.