The great thing about 4th Edition is that the character choices provide players with several options. Whether you play a wizard, an avenger, a bard, or a fighter, you can choose from a number of resources with varying costs on your turn—usually graded along the spectrum of abilities that can be used every round, once per encounter, and just once a day.
The 3rd Edition rules, building on the concept of non-weapon proficiencies from 2nd Edition, introduced hard-coded options into every character class via skills. Players with wizards and fighters alike chose skills during character creation, though usually they chose different ones. (Skill selection is, of course, also part of 4th Edition.)
1st Edition characters used a core set of rules and a matrix for determining the success of attacks and saving throws, but for the most part, characters of one class didn’t necessarily access their abilities like characters of another class. The game mechanics behind how a wizard cast spells were in no way similar to how well a thief could hide in shadows, for instance.
With that brief overview in mind, now consider how many options a given class has when compared to the complexity of other classes within the same edition. In other words, compare the complexity of a fighter with a ranger or a wizard.
For example, a 1st Edition fighter essentially chose a weapon and armor, and then was good to go. Compare that to a 1st Edition paladin, who could detect evil, had bonuses to saving throws, could lay on hands, cure disease, and so on. Then we have a 1st Edition rogue, who could draw upon a specific set of abilities described on a table to pick pockets, open locks, find/remove traps, and so on. Wizards and clerics could choose daily spells to prepare, while most other classes had only a few, if any, daily resources.
Ultimately, the philosophy on character complexity between older editions and 4E is starkly different—earlier editions gave some classes far fewer options than other classes. Such classes are generally regarded as easier to play, or from another point of view, more open to improvisation. On the other hand, 4th Edition classes are superbly and obviously balanced against each other, and though no class is easier to play than any other by a large margin, each class does provide every player with a robust list of choices for engaging with the game.
Which of these is most important to you? (Choose only one.)