Know your opponent's style and use it against them

I've noticed that some people have different styles and you can turn that against them sometimes.....there was one guy at our club that always used knights to full advantage....I just learned to trade the Knights early, then he was lost most of the time. There was another guy a number of years back that was rated about the same as me at the time (1500ish) that I almost always lost to, but he ALWAYS played the French as black....I just learned how to play the French and started beating him most of the time. There was another guy that was a crazy attacker...always attacked whether sound or not....just had to learn to play solid against him and be patient, run him out of pieces and win the endgame. Would like to hear other's comments???

You mean weaknesses? Non-master players, and even plenty of masters, have no style. They just have weaknesses. 

You need to be a decent all-around player before you have a style.

For instance, I would say I have a defensive/counterattacking style, but the truth is I'm just terrible at attacking, and less terrible at defensive/counterattacking play.

I'm not sure.

I like to think of myself as flexible. I'm willing and have played sacrificial attacks OTB. I'm willing and have played fortress-like defense where I have no threats. Technical endgames, theoretical openings, silly openings... open positions, closed positions...

But I wonder if it's a matter of perspective. Maybe if I played a 10 game match vs a 2400 person, they would say I'm terrible at this or that, so as long as that's present in the position they can always beat me.

Things I probably struggle with are breaking principals in a big way... like giving myself terrible structure or a weak king for some kind of dynamic compensation. Sure this somewhat easy if you get a big initiative, but something more subtle like "my pieces are more active" would be difficult to even consider in a serious game.

It's a matter of perspective. At the 2000 level of consideration, I'm clearly strong in some areas. But seeing the poor moves I play according to stronger players and the engine, I'd like to assume otherwise. 

 Yeah, OP is basically saying some people are a one trick pony. Don't let them do their trick, and they fall apart.

Chin basically got it....but at the higher levels people have less weaknesses or a more universal/less pronounced style (it is just semantics like every step says), they know when to attack and when to defend, so maybe applies less at higher levels....still I see in many lists of GMs a "style" listed next to their name....things like "attacking", "universal", "tactical", "defensive", etc....for instance Steinitz is known for his defense....Tal is known for his masterful tactical attacks....etc....

and Cherub is known for his Bishops.....lol.