As usual, no one really seems able to agree on exactly what degenerate play is. For me, it generally means playing against the spirit of the game or good sportsmanship. If I want to smack you, it might mean that you are engaging in degenerate play. Or, it might mean you are just playing better than me :P
If it's an intrinsic flaw in the game, that falls under a game being broken or solved. I don't think many people would call putting an X in the middle of the Tic Tac Tor board degenerate play.
I'll be honest, many examples of degenerate play, like only buying silver in Dominion, really seems like a learning curve. As players gain more experience, they learn overcome the low plateau strategies.
In all honesty, the games with actual degenerate play that doesn't either involve people needing more experience or the game actually being genuinely broken are games like Magic the Gathering. When every card adds or changes rules and there are literally thousands of cards, there end up being combinations and loopholes beyond what the designers anticipated.
Really, the bad sportsmanship part feels important to me. If someone's gets upset or their feelings get hurt, I think that's significant. Particularly when someone is doing it for their own entertainment.
For me, that is the real problem with degenerate play. When someone's intentionally subverting someone else's fun. If a group of friends want to sit down and have fun with loopholes, I'm not going to find fault with that.
Although all that gets turned on its head when there's money involved.
Tournaments, when there's a purse involved, aren't about fun. Is degenerate play ruining the environment or is it educating the developers or is it just playing smart so you can win? Is there a line where it counts as cheating?
There's definitely a line but I don't know where it is. I don't fault playing smart. I don't fault playing vicious. I don't fault playing to win. But somewhere beyond all of those, there is playing to hurt. Maybe that can be degenerate play. Maybe that's just being a jerk.