In 4e, ability scores can improve somewhat rapidly by means of natural advancement. As always, the player has some choices to make, because - while all scores improve eventually - only the top two will do so very quickly. It can be expected that primary and secondary abilities will be increased at the rate of 3 points per tier (at every opportunity), whereas others will only increase 1 point (at 11th and 21st). This will lead to a gradually growing gap between a player's strengths and weaknesses.
Ability scores contribute to the following:
- Attack rolls,
- Damage rolls,
- Skill and ability checks,
Any given character will have access to attacks dependent upon only one or two abilities (assuming they don't go out of their way to mess that up), which will most likely be their primary and secondary scores. Given this and the relative lack of arbitrary attack roll bonuses, their chance to strike is relatively predictable. At any level of gameplay a +/-1 to attack will have the same degree of effect, which is to say, bonuses should not scale. Small bonuses or penalties prove to be a welcome boon or challenge, but large ones, it is safe to say, will always threaten game balance. Attack rolls and the defense of monsters should increase at roughly the same rate, assuming one desires the same relative difficulty level throughout the game.
Damage, much like attacking, is usually based on either the primary or secondary ability score. As such, it's progression is similarly predictable, as there should never be a case where tirtiary scores must be used. Damage should progress at a rate comparable to monster hit points, in the ideal scenario, to ensure difficulty remains constant.
Peculiar among defenses, armour class is affected by an ability score only when the player is lightly armoured. Use of heavy armour removes its influence. On the surface, it's affected by your reflexive abilities (Int and Dex), but many class features - especially on lightly armoured classes - modify this, allowing the character to use a primary or secondary ability. Ideally then, heavily and lightly armoured characters should see their AC increase at a proportional rate, so as to prevent a growing gap between them toward the later levels.
The three defenses rely on three different pairs of abilities which, to my knowledge, are not altered by any effects. As such, only one or two will be covered by your primary and secondary abilities. The natural result of this is that one defense will gradually fall behind as players approach higher levels.
More on this later... sorry, pressed for time.