Let's be careful here: It's not necessarily a concert foul when one guy says to the other in passing, "Hey, that's a nice Ibanez guitar!" or "Is that a Big Muff pedal?"
No, things go afoul when this dude decides to prolong the conversation. Unfortunately for you and everyone around you, the conversation doesn't stop when the band starts playing. And it doesn't help that the guy sounds more and more like he's full of shit the longer he talks.
Imagine trying to enjoy a band's set. You're taking everything in -- the band's performance, the audience reaction, the balance of sound and visuals, the general vibe in the room. And then you hear a one-sided, shouted conversation going on behind you that might sound like this:
Guy: "Jesus, those tubes in his Marshall amp are humming like a motherfucker! He's just ripping on that Les Paul! And those Ernie Ball strings bend soooo fuckin' well! And listen to his processor! Digital tone gold!"
Guy's Friend: "Yeah..."
Listen: there's nothing wrong with being into equipment. Far from it. The thing is, not many people go to your average live show and want to talk shop. There are guitar expos every once in a while for that sort of thing. And there's always Guitar Center, too.
Let's face it: Equipment doesn't necessarily make a musician play better. Endlessly babbling BS about equipment doesn't make the show a better experience either.