Nail said:Tumble, IMHO, is not such an ability.
To me, Tumble (and Casting Defensively) are exactly such abilities because the reason AoOs were introduced in the first place was to prevent non-stoppable actions in the game (due to a segmented circular initiative system), but the mechanics of these types of skills are such that PCs (and NPCs) can get around the AoO mechanic completely, especially at mid to high level (and even very low level for both Tumble Past and Cast Defensively).
Hence, Readying tends to be the only reasonable solution at high level to prevent Tumbling Rogues from getting past the guards and killing the King.
In fact, with the introduction of Immediate Actions (to also resolve problems with the segmented circular initiative system), Fighter types get screwed over even more.
Spell casters are more capable of reacting outside their turn, but the only mechanic that Fighter types had for reacting outside their turn was AoOs. AoOs are pretty much watered down with the Tumble Skill, the Casting Defensively skill, that spell in Spell Compendium where an Archer can fire missiles without provoking AoOs (forget its name).
As more books come out with more Immediate spells and more feats like the Improved Counterspell type feats, Spell casters are more capable of reacting outside of their turns, even at real low levels. As levels increase, Fighters are less capable of reacting outside their turns.
That is practically the definition of lack of balance.