In 2e Dark Sun, it was assumed that characters would need to make several characters (a character tree), all at least 3rd level, due to the fact that Athas was so deadly.
They have removed that ruleset in 4e due to the fact that:
a) 4e PCs are generally tougher.
b) Themes give characters an extra encounter power, so it makes them roughly equivalent to a 3rd level character (other than the to-hit bonuses and hitpoints)
and c) While, thematically, 4eDS is very similar to 2eDS, the newer version focuses more on the fun factor as opposed to the "We'sa alls gonna die'sa" factor. It is still a very deadly world, but much more manageable now.