As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
You have one false assumption in your question: what you observed was not two quests, but rather only one.
In other words, the quest was "Capture Point X, while defending point Y from Capture". The quest lasts until one team manages to capture the other, which means, in your case, the 'defend' portion of the quest lasts until your team manages to capture the enemy point in question, rather than a simpler, "defend for X".
You could theoretically have to defend that point for the rest of the game, as long as neither of the two target points changes hands.
Best Answer
Dodge now been removed from the game.
Dodge items do not stack additively. For example, Jax, with level 5 counterstrike, has 18% dodge chance. If he then picks up ninja tabi (+12% dodge) his total dodge chance is 28% (and not 30%).It's easiest to simply calculate the highest %dodge first (as they stack multiplicatively, order is actually irrelevant): In our case, the 18% dodge chance from counterstrike is simply added (18% of 100% hit chance is 18%) so Jax's dodge chance is 18%, and his "be hit" chance is 82%. Next, the dodge chance from ninja tabi is added. 12% of 82% "be hit" chance is 9.8%, which then gets added to the previous 18%, for a grand total of ~28% dodge chance.
If there were another dodge item (say, an identical version of Ninja Tabi that stacked with itself), the +12% dodge would be 12% of 72%, or 8.6% increase.
The reason dodge is handled this way is such that the mitigation of dodge as a stat scales linearly; were dodge strictly additive, the more dodge% you had, the more damage further dodge% would reduce.Ignite
Casting ignite on an already ignited target will reset the duration of the DoT, losing any remaining damage the original ignite would have done.
Speed
I'm just going to quote from the "mechanics FAQ" from the official LoL forum.
And one addendum: The strongest slow/speed boost is always applied first.
Lifesteal
Lifesteal has no diminishing returns. If you hit an opponent for 20 damage, and manage 200% lifesteal, you will regain 40 health.
Lifesteal is based on damage done, so it is reduced by Armor (which does not have diminishing returns, see below) and doubled (or more!) by crits.
Attackspeed
+Attackspeed from items is additive and references the base attack speed of the champion. Buying two phantom dancers, for example, gives you 110% additional attacks per second, but because they are additive, the effect doesn't increase due to any other items you may have.
Attack speed has a hard cap of 2.5 attacks per second.
Armor and Magic Resistance
The effectiveness of Armor and Magic Resistance do not suffer from diminishing returns.
Please see this answer for a fully detailed explanation of why.