RANGE AND FLEXIBILITY: The range and flexibility of the game has been significant reduced.
(1) Although you can now go from 1st to 30th level, the scale of actual power wielded by your characters is significantly smaller than it was in previous editions. Both the low-end and the high-end has been lopped off.
(2) You have far less ability to customize your character.
(3) There is a much narrower range (an almost nonexistent range) of play-styles supported. In “Death of the Wandering Monster”, I talked about how there was a huge difference in previous editions between the ways in which clerics, fighters, rogues, and wizards played (for example). This could lead to “balance” problems if a particular group’s style of play catered to one style of play over another, but it also meant, in my experience, that different players gravitated towards their preferred style of play and, if they got bored with that style of play, they could switch to another style and keep the game fresh.
4th Edition, on the other hand, only offers different gameplay within the context of combat. And, even there, the differences are not as significant as in previous editions.
ROLES AND CLASSES: On this topic, however, I also want to suggest that you consciously toss out whatever preconceptions you may have about how the different classes play based on previous editions. They are almost assurredly wrong.
I also want to encourage you to go one step further and toss out whatever preconceptions you may have formed about how the different roles will play.
For example, in our experienced gaming group we saw that fighters are defenders. Based on how fighters had played in previous editions we had, unconsciously, ended up with some preconceptions about what it meant to be a defender and how a defender should be played.
We were wrong. Not horribly wrong, but wrong enough that until we sat back and re-analyzed our preconceptions the group was meeting with some frustrations.