Why is hitting "ignition" such a big deal for fusion energy? There was some news lately about a reactor hitting 70% of the way to ignition, which I took to mean it output 70% of the energy which was put in.
Is the 100% point just a interesting milestone, or is that where fusion becomes economical? Is the fuel so cheap that it'd be profitable to run a reaction where 99.9% of the energy was used to sustain it?
 A: So there's two main terms in (commercial) nuclear fusion: breakeven and ignition. They correspond to a $Q$ factor for the system of $1$ and $\infty$, respectively.
For nuclear fusion to occur, you have to heat the plasma. You have to put enough energy in so as to overcome losses (e.g. to the surroundings) and so as to still deliver the required "heat" to the plasma.
Once fusion starts happening, some energy is released.
Breakeven is when $P_{\text{put in to heat & keep plasma going}} = P_{\text{released from fusion reactions}}$.  That is, you are now "covering" your losses, and any energy you generate further to this is a net energy gain.
Ignition is when there is so much energy being released from the plasma, that this is heating itself. You don't need external (expensive) machines to keep heating the plasma to ensure fusions reactions occur. You can turn everything off and it would still be going. It is self-sufficient, and hence producing the largest possible amount of commercially useful energy since you don't have to "lose" any of it to put it back into the plasma.

Is the 100% point just a interesting milestone

Well, always aim for the sky.
I don't know if it'll be possible or even safe to have it exactly at $100 \%$. It would certainly be a great demonstration for commercial energy production.
