A major feature of MW2 gameplay is the frequent use of the jump jets. A DFA is not hard against a non-maneuvering target, but the real challenge comes with a maneuvering one. If you've never done a DFA, I can describe the appeal in three different scenarios:
1.) The unaware opponent (e.g., radar off) with his freshly spawned, undamaged mech is walking along, oblivious to your long, high, jump-jet ascent, when.... WHAM! -- he's instantly destroyed when you crash at a couple hundred kph onto his head.
2.) The fully aware opponent, damaged after an extended fight and with depleted jump jet juice, tries desperately to move away your descending feet, but... can't. Boom. It's the near inevitability of it that is the appealing part here, a sort of "I know what's coming, but maybe I can just move... *CRUNCH*"
3.) The aerial duel, in which two mechs square off in the air, one rising to meet the other, ends when either a.) the rising mech smashes into the aloft mech's feet (humiliation!), or b.) one of the mechs falls first (after a flurry of fire) and the other has enough maneuvering ability left to come crashing onto his head.
I'm sure others can describe more situations by recalling some of the great narratives that MW2 gameplay naturally builds. MW2's DFA is really one of the crowning glories of the game, one of many afforded by Repomancer's awesome engine. There's nothing quite like it anywhere.
Some pictures:
Here's a #2. Note my speed: I'm desperately trying to avoid it --
Here's another #2 --
Here's yet another #2. Yeah, I guess I legged him and then DFA'd, a bad show, but I did take some splash damage (last pic)

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It's a lot harder to capture the others when they're happening.