The differences are massive.
Just for a start, you can be an absolute beginner and start an AFF game in 10 minutes: rules are very light and straightforward, highlighting cinematic aspects of your game.
The basic mechanics of this ruleset hinge on 2d6 rolls and melees are decided by opposed rolls (with various modifiers), rather than by a d20 roll vs. Thac0, meaning that if you are inexperienced but with a thick armour and fight with swords versus an experienced fighter in his shirt, you will probably feel the pain, as it often happens in real life.
What I like of the 2d6 mechanics is that the mid-way result (7) is much more likely to happen than the extereme results (2 and 12), that again to me is more realistic: if you try to make a series of long jumps, you will jump more often an "average" distance than very long or very short, while with d20 any result has the same chance of being rolled up: 1 and 20 have the same probablility of happening as 10, for example.