I read these books ages ago, and there was a good reason to my reading so many: the first one ruled. The first book is significantly different from the rest, and here are the differences:
An awesome villain: Cluny the Scourge was scary as hell. Later villains almost wet themselves when they come face-to-face with the heroes.
Humans: Yes, there were humans in the original. There are many hints to their existence. Later books cut this out to make the fantasy world more appealing to little kids.
Different sizes: The animals in Redwall were almost to scale (though obviously some were shrunk/grown quite a bit). Later on this size difference is almost non-existent, another factor that makes them more like people in animal suits than animals in people clothes.
Minimal likkle critters: The "Dibbuns" are effective only if you just ate something nasty and want to get rid of it. They dominate later books, but are a much smaller presence in the original.
Lack of heroes: The heroes in Redwall were a lot more unassuming than the later heroes. Matthias was a clumsy monk, Basil was an insane old hare... even Constance, the best fighter of the lot, was old and grey. Later books had soldiers and Badger Lords in plenty.
So don't blame me for being proud of this book. It's a good 'un.