After DM'ing for a few months, I finally got to *play* a session of
4E at the weekend so I feel more confident in saying this. For me, it's still being able to play encounters to their fullest, because everything the monsters can do is RIGHT THERE in front of me. Synnergies are simple and clear. Abilities are summarised in a few words. Simple mechanics like the recharge and saving throw rules keep things uncomplicated.
Previously I felt I had to know the game inside and out to make the most of an encounter, something which my limited time (and, frankly, capacity to remember the minutae of the rules) made very hard. Things are very different now, and the game is much more enjoyable for it.
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"The last time I ran into myself, I kicked my own ass."
Chasing the DM, a blog for DM's like me who really feel they should know what they're doing by now.

For DM's:
4E Dungeon Index (adventures, conversions, and sidetreks by level, last updated 16th Oct. through Dungeon #171).