I've heard a number of people say the beginner box is excellent. What is it that sets it apart?
It's a few things:
It starts with the basics and goes from there. It strips out a lot of the less simple aspects of the game (no AoOs, no combat maneuvers, etc).
It starts you off in a 1-player adventure and walks you through what you have to do and how to do it.
For character creation, it strips out the vast majority of options and gives you static selections. For example, there are only three wizard options instead of all the different schools (universalist, evocation, illusion). The wizard also doesn't get a familiar, he gets the bonded item option instead.
It doesn't have item creation to deal with or understand.
The only classes are the "core four" - fighter, cleric, wizard, rogue
The only races are the dwarf, elf, and human
It gives suggested feats, skills, and equipment to make character creation faster.
It has a very conversational tone and does not assume any knowledge of how the system works.
The racial entries are simplified. For example, dwarves don't have Defensive Training, Greed, Stability, or Stonecunning.
The classes aren't written in table format - it's "1st level - here's your crap." then there's a level-up section that deals with what you get if you get to 2nd level.
There are under 30 total feats to select.
There are no metamagic feats.
There are no exotic weapons.
There is no encumbrance (but there is slow down from armor).
Lots of things like that - things that you would likely want in a full game, but removing them from a "Beginner" game makes it a lot easier to understand and quickly play without having to do hours of research. You can have a character ready in less than 10 minutes.