As this is the first room that and adventuring party sees, it can go a long way toward setting the tone for the dungeon. I like my entrances to be interesting, but as this room is often the most frequent through-way for adventurers going in to plunder the depths and monsters sneaking out to raid the countryside, it's unlikely to be filled with interesting items, traps or tricks. By the same token if the room is a 30'x30 square with nothing in it but doors or archways and a dusty floor, it serves as a rather drab opening for the "mystical underworld".
So, perhaps it is best that the entrance leads to a corridor, allowing the players an initial choice and some variability around which room the party encounters first. With this approach, different parties can find different first rooms. Of course this does little to help capture the players imagination initially. "Ooooh a 10' wide corridor! How novel."
In the first edition Dungeon Master's Guide, Gary Gygax presents a small selection of dungeon entrances to be used with the random dungeon generation tables in the appendix. While these are useful, I've never found them to be particularly inspiring, and I'm a strong believer that maps should be inspiring. I think that a GM should look at a map and think, "now that's a cool looking dungeon!" But this is heading toward a discussion of how important are cool maps to cool dungeons, with is a topic for another time.
