Even the way Onslaught is structured feels like a remix rather than a whole new song. It's basically Rush crossed with Conquest, as you push forward towards an enemy stronghold and then play Capture the Flag for a few minutes while the timer ticks down and the flag rolls up. These are tried and trusted design choices, but they never combine to justify the overwhelming panic that the word "onslaught" promises.
For one thing, you're the ones doing the onslaughting, which feels upside down. If I'm playing something called Onslaught, I want my back to the wall, I want impossible odds and I want to know that my survival comes from basic combat skill, not just the fact that I've played the map 500 times before and know exactly where to hide.
The Halo Firefight and Gears of War Horde template may soon be as overexposed as Capture the Flag, but the prospect of playing a "last stand" scenario with the power of the Frostbite engine's environmental destruction is incredibly appealing. Knowing that your fortress is being chipped away with each explosion, forced to react to attacks from all sides - Onslaught only taps into that buttock-clenching primordial defence impulse for those brief flag-capturing stand-offs before you're on the move again, and that's a shame. You can already experience the hunt-and-evade thrill against live human opponents, so swapping them for the more predictable challenge of AI bots doesn't elevate the formula at all.
That's not to say Onslaught isn't tough. Hardcore difficulty is incredibly punishing, with your health halved, your guns weakened and your enemies apparently upgraded with bulletproof heads. It's a cheap way of cranking up the challenge, and victory comes just as much from stubborn determination rather than the application of superior tactical co-operation.
The absence of something like Left 4 Dead's AI Director is sorely felt. While the combat is as muscular as always on a moment-to-moment basis, it lacks the sadistic sense of evolving shape and pace that Valve has injected into co-operative play. Making the gameplay peak and trough depending on how the participants are playing, rather than which bit of the map they've reached, would go a long way to making Onslaught feel more essential, more alive.
Bot-bashing has long been a part of Battlefield's heritage, of course, so it's hard to blame DICE for wanting to revive the tradition for console gamers new to the series. It's also hard to forgive the decision to price such a slim addition to the game so highly, especially since juggling existing game assets to come up with new twists is the sort of thing that used to be supplied for free by the mod community.
That's not to say that DICE and EA aren't perfectly entitled to charge money for new menu items, or that every DLC offering should reinvent the wheel. Bad Company 2 remains a superb shooter (if still rather borked by that last patch), but having gorged on it for three months we need something more interesting than second-hand spaghetti bolognese if we're expected to pick up the tab.