Technically you do need to confirm a critical hit. It is only a critical if the roll of a 20 plus the modifiers would result in a hit. To me this is somewhat more reasonable. Basically what it is in my mind is rarely when you are fighting, you'll do something really well (nat 20). This is always a hit, no matter what (so you could be facing one of the best guys ever, but you'll still hit), so to somewhat balance that, they make it a crit if you would have hit already (to make up for the fact that you didn't really need the auto hit). Plus the critical hit is not nearly as potentially devestating as before (rolling the damage again). To me the idea was make critical more of a "really good hit" not necessarily a "critical" hit. In the end, I think it largely averages out (with the possible exception of really good rapiers with extended crit ranges from 3.5)...
There is a lower overall damage possibily at the lowest levels of Heroic Tier, when compared to 3.5 Critial hits. A 4E crit does maximum on any dice you rolled with the attack, weapon dice, power dice, and class feature (ie sneak attack) dice. Where it really changes and 4E critical hits become something quite stunning, is with magical equipment and some class powers.
All weapons with magical bonuses grant bonus dice to the damage on a crit. These additional dice aren't automatically at maximum and their size depends on the weapon (a basic enchantment gains 1d6 per +1 on the weapon). Similarly, if you are using a weapon with the "High Crit" quality than you can get alot of dice.
A 4E Fighter at 1st level with a longsword rolls roughly 1d8+4 for damage (+5 if he took weapon focus or whatever else, there's lot of ways to get more damage in 4E). So, on a crit that 1st level Fighter does 12 damage.
If that Fighter had been using a Scimitar (1d6+4 normal damage), which has High Crit, than a critical hit would do 10 + 1d6 because High Crit allows you to roll the base damage die one addition time per Tier (Heroic=1, Paragon=2, Epic=3).
If the Fighter keeps the Scimitar and then gets an enchanted version, let's call it a +1 basic, than on a Crit he would get 10+2d6, one for the High Crit and one for the +1 on the sword.
And it only goes uo fromn there.