I also have a problem with natural 20's being auto crits, but by looking at the way the rules work, it should become clear that this will actually come up far less than it would in in 3e because of the monster creation rules and the way bonuses scale.
In 3e if you wanted a bunch of mooks to fight, you had to use lower CR creatures, meaning their attack bonus's would be much lower, often requiring close to 20 to hit, which, without conformation rolls, would be a problem (and occasionally the same problem in reverse when you create a BBEG), however, proper creation of minion and solo monsters in 4e should prevent this becoming a problem.
But what if you intentionally want to put the PC's up against some really crappy or really grubbly guys you ask? Remember that bonuses scale by +1 per two levels, meaning it's quite possible that by the time attack rolls require this amount, the disparity in hp, damage and other abilities will be so much that the difference in power may be significantly more overt than in 3e.
Well, that's pure speculation, but the minion thing is certainly going to help alleviate the problem.