The short version is that you want to avoid flying on DL metal whenever possible. The longer version is that, in my personal experience, they run a lot of aged 767s with uncomfortable seats, they serve undrinkable wine and inedible food, and they will happily screw you if something goes wrong, blaming their subcontractors. And that's in business class as FB Plat. In back, you get an even cheaper version of all that, plus the corporate culture of lighting the seat belts at the first sign of light chop. If you're FB, you're flying DL over the North Atlantic, so good luck going to the bathroom with fifty other people in the thirty minutes allowed.
It's funny how, at least on the routes I monitor, AF/KL sells out way before DL does.
Okay, they're not as bad as American, but "we're a US airline that maintains its aircraft and usually dispatches on time" is hardly a marketing slogan.
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The Platinum benefit on DL is the experience you gain from knowing to avoid flying DL and the stoicism for enduring it when you can't avoid it. It's kind of like using a Turkish public toilet, but saying so would be an offense to Turkish public toilets.
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