Guild Wars 2

I think I remember reading somewhere that after a certain amount of toughness, it starts losing its effectiveness, not sure if this is true or it applies to all classes the same if it is, was hoping someone knew these numbers so it can help me plan out what gears to get since I finally just got to 80

Well as a thief, I much prefer vitality over toughness. It just seems to be much better overall for me. Especially when it comes to conditions.

http://www.guildwars2guru.com/topic/60838-math-damage-reduction-toughness-and-vitality/

Damage reduction ratio, R
= X/(A+X)

X = Toughness/armor you want to add
A = Current armor value

Let’s take base toughness+armor or a warrior; 2127 and assume the output of the damage formula was 2,127,000 to make the math easy. You will take 1,000 damage from this hit. (2127000/2127).

It is clear that adding 100 more toughness results in the formula changing to 2127000/2227, or roughly 955.
It is then clear that adding 200 more toughness results in the formula changing to 2,127,000/2,327, or roughly 914.

It is then clear that adding 300 more toughness results in the formula changing to 2,127,000/2,427, or roughly 876.

We can see adding 100 toughness reduced our damage taken by 45, a further 100 reduced it by another 41, and the final 100 reduced it by a further 38.
This is the diminishing returns you think you are seeing.

However, turn that into EHP terms…
If you ONLY had 1000hp and base toughness, you would die in 1 hit.
If you had 1,000hp and +100 toughness, you would require 1.047 hits (1000/955).
If you had 1,000hp and +200 toughness, you would require 1.094 hits (1000/914).
If you had 1,000hp and +300 toughness, you would require 1.141 hits (1000/876).

And you can now see that toughness scales the same way for survivability as power does for damage: linearly.
The major difference between the two is that you can double your base power (resulting in double damage) by simply adding 916. The same does not apply for defense because it is made up of 916 toughness (which you can double) and a further ~1000 or so from your profession’s armor class.