Well, one start is on a very basic level: Ammo.
The 5.56mm, due to it's high velocity, when fired at close ranges such as those found in the close quarters street fighting, passes right through the target. As such, it fails to achieve it's major damage factor ; the "tumble effect" that makes it effective at longer ranges. If it's passing right through the target without inflicting it's devastating wound channel via the "tumble effect", it's not doing much of anything, unless it's going through a person's head or heart.
Same with the 9mm NATO pistol and SMG round. It's got a high velocity, it's lightweight and you can carry lots of it. Problem is, while you can take an M9 into the field with 15+1 out of the holster, the high capacity of ammo is meaningless when you're lying on your back in a pool of blood, shot by a guy you put three 9mm rounds through that didn't stop him.
It's an ironic reverse of the lesson of The Moro Insurrection that led to the development of the .45 ACP in the first place, the round the 9x19mm NATO replaced.
So, what the military should have taken away from it was: We need bigger, heavier bullets that transfer as much energy to the target on impact as possible to ensure that what goes down stays down.
That's the point at which the US should have begun exploring a new intermediate range infantry round and re-introducing the M14 (in semi-auto, as a designated marksmen rifle, which, to some extent we have done since) and re-adopting the .45 ACP as the standard pistol/SMG round.
Ironically, it wouldn't be until the eve of the resumption of hostilities in Iraq (2002) that work began on the 6.8mm Remington Special Purpose Cartridge. Eight years on and, despite performing beautifully on every platform I've ever fired it from, countless demonstrations of it's superiority in knockdown/stopping power over the 5.56mm and a serviceable (and cost effective) replacement system for converting the existing stocks of M16/M4 family of weapons to 6.8mm SPC (The Barrett REC7; upper receivers are fully compatible with existing M16/M4 lowers and 6.8mm SPC is compatible with existing STANAG magazines used by said weapons) at a reduced cost to the tax payer and a not insignificant savings for the DoD that could be committed to other projects in need of funding.
So, on that score, more effective ammo, we HAVE the answers, but we're still sitting on the same insufficient weaponry that isn't doing it's job.