The enabling techs for power armor would be a compact power source and compact actuators putting out significant force. The protection part (various ballistic plates stuffed in kevlar pockets) are already in existence. The sensing parts (IR cameras, binoculars, light amplification) are in existence as well, while communications have already miniaturized.
Current power sources are inadequate for the task because batteries are heavy for their power output and have to be recharged often, while internal combustion engines are easily detected. Current actuators, e.g. electrical motors and step motors, have to be made big and heavy to get the kind of force or torque output necessary.
From a military perspective, if the armor is not significantly bigger than a soldier, while offering superior protection and strength without impeding mobility, it is difficult to see how the armed forces would not be all over it. Note that the chief complaint to current armor is that is it too heavy, rather than too bulky. In a power suit, the weight problem will be solved. Also, as best I can tell, losing limbs currently happens due to shrapnel wounds or due to outright tear by the force of the explosion. A power suit with interlocking plates covering almost the entire body would stop almost all shrapnel, while the underlying strong mesh of the exoskeleton will keep the limbs from being torn off. At least as far as the plates are concerned, we know it works, because soldiers have a much higher rate of survival nowadays exactly due to armor protecting the vital organs in the torso.
The counterargument of research expense is not valid, IMO. We spend over 100b per year in Iraq. I think the armor can be researched for a lot less than 1b in total costs. A modern warship costs more than that just to build. Also, the research is a one-time investment, rather than a recurring cost. Add to that the hundreds of patents on enabling and lateral techs and the resulting companies and tax revenue for the state, and the gov may even get out ahead on that.
The counterargument of countermeasures is literally valid but ultimately impractical. There will always be countermeasures. Just because there are shaped charges, RPGs, and kinetic missiles, we have not stopped making/using tanks, and just because there are SAMs, we have not stopped making/using aircraft. Their use is affected by the countermeasures but by no means negated or forestalled. Yes, if the terrorists cram a van full of explosives and ram it into a GI in power armor, the GI will not make it, but so what? If that would be the only way for them to kill our soldier, the armor has been astoundingly successful.
Finally, a tech like power armor has significant benefits beyond the physical. Decreased casualties sap the morale of the army and the society at a far slower rate, while the enemy is both frustrated and intimidated by his shrunken ability to be effective in combat.