The biggest improvement was the adoption of Ceramics armor starting in the 1970s. The problem with such Ceramic Armor is the price and the need for it to be flat. Look at the M1 Tank, it does not have any curves, the Armor has angles but no curves for this reason. This seems to be followed by the MRAP, i.e NO curves, but all flat armor. Thus it may have Ceramic armor, which is stronger than the Steel armor of WWII, but not that much. You talking about no more than a 10-20% increase in strength which means they have better armor than most of the tanks from the earlier years of WWII (Most were all un-armored) but the Armor protection is less than the Tanks of the middle of WWII and the end of WWII (i.e. the Sherman, the German Panzer Mark IV, Panther and Tigers and the Russian T-34 tanks). No one is saying the MRAP's armor is better than the Armor of the Sherman let alone the T-34 and Panthers and all three could be taken out by the WWII Bazooka. The Improved 3.5 inch post-war Bazooka was design to be able to take out the much heavier Tiger and JS-III tanks. The 3.5 Bazooka was replaced by the 66mm Light Antitank Weapon (LAW) during the 1960s, which could take out the Soviet Tanks of the 1960s. The Soviet RPG-7 was design to take out the M60 tanks of the era. The RPG-7 is still in use in Iraq along with its successors. The RPG-7 is marginal against the M1 tank, but it is effective against anything lighter such as the MRAP.
Anyway one of the problems of increase armor is increase fuel usage along with increase fuel usage by its support elements do to the fact the support elements must upgrade its wreckers and other equipment to be able to support the MRAP. I just do NOT believe we can provide the fuel to these vehicles, for if the US military could do so, the M113 would have been pulled out of retirement and used (People in Iraq were actually demanding them knowing what depot the M113 were being kept in).
http://en.wikipedia.org/wiki/Image:Force_Protection_Cougar.jpghttp://en.wikipedia.org/wiki/MRAP_%28armored_vehicle%29http://en.wikipedia.org/wiki/Image:FPCougar.jpgOne last comment, did I wrote 60 Tons for a Sherman? That is to high, the Sherman weighed 30 tons (This was do to transportation limits of shipping the tanks overseas as oppose to any lack of desire for heavier tanks, when this shipment limit was removed in the 1950s US tanks grew to be over 60 tons). The Vietnam Era M60 Tank weighed 60 tons (as did the WWII German Tigers). The M1 weighs over 70 tons. Despite this the real comparison is between the MRAP and the WWII Sherman. The MRAP weighs 2/3 of a Sherman, its Armor may get up to Sherman levels with the use of Ceramic Armor, but that still means any Anti-tank weapon designed since WWII can take it out. The MRAP is NOT a Tank, and as such does NOT provide the protection a Tank does to its crew (and the M1 Tank does NOT have the ability to haul Infantry in excess of its own Crew).