What is a breakpoint?
A breakpoint means a value at which certain abilities experience a "jump" in strength. In most cases, this refers to having enough haste rating for Damage-Over-Time (DOT) spells to gain an extra tick.
For example, suppose you are a fire mage using Living Bomb. If you have 13163 haste rating (37.5204%), your Living Bomb would tick for 6 times. If you had 1 less haste, at 13162 rating (37.5179%), your Living Bomb would only tick for 5 times. That 1 haste rating causes your Living Bomb to tick once more, and hence become quite a considerable amount stronger.
Are "breakpoints" the same as "soft-caps"?
No. A "soft-cap" is the point at which certain game mechanics make a stat much less valuable. For example, all spells have a "haste soft-cap" when the cast-time or GCD reaches 1 second. The "Shatter" mechanic creates a "soft-cap" for Critical Strike rating, when your spells are guaranteed to Critical Strike against a frozen target.
Breakpoints and soft-caps are very different. A breakpoint is a requirement. Think of height restrictions for at theme parks; If you're not tall enough to pass the bar, you don't get to join in on the fun. Similarly, if you don't have enough haste to pass a breakpoint, you don't get the extra tick. A soft cap on the other hand is a limit. Think of say, the peak of a mountain. You climb until you reach the peak, but then you stop - because if you keep going, you'll be climbing back down. Similarly, you want to stack a stat until you reach the soft-cap, and then you stop. The main difference here is:
You want to PASS a breakpoint. You want to REACH BUT NOT PASS on soft-caps.
Aren't they all gone in Warlords of Draenor?
No. Most Breakpoints will be gone in Warlords of Draenor with the new DOT system. On the other hand, most soft-caps may have their values changed, but will be staying.
Is 14242 haste a breakpoint (for Frost Mages or Frost Armor users)?
No. 14242 haste rating is a soft-cap, not a breakpoint. 14242 haste combined with Frost Armor and the raid 5% spell haste buff, gives a Mage exactly 50% spell haste. This reduces our Global Cooldown (GCD) down to 1 second. There is no magical gain you get at 14242 haste.
Are breakpoints important for Mages?
No. The main spells for Mages that are affected by breakpoints are Mage Bombs, and while they do benefit, the gains are really small and not worth going out of your way to reach. I will elaborate on this in a future TC post.
What about soft-caps?
Yes. This applies especially to Frost spec. The two main caps we care about are 50% (1 second GCD cap) and 100% (Frostbolt/Arcane Blast/Arcane Missiles 1 second GCD cap). The first one we can reach passively using Frost Armor and haste rating from gear, while the second one only concerns us when under haste effects such as the legendary meta gem or Time Warp/Heroism/Bloodlust.
Will soft-caps be a big concern in WoD?
Maybe. As of the time of this post, it's too early to say. With the removal of reforging, we will likely have less flexibility to deal with soft-capping, so we can only hope that stats won't easily reach levels where soft-caps really hurt us.