What is the difference between a 10Gb LR optic that can only go 10km compared to a ZR optic that can go a lot further at 70km?
ZR optics are spec'ed to 80km but they're not part of the 10GE standard; they're rather a "Cisco special."
In terms of what actually allows them to transmit light over this distance may be "protected" information from Cisco (both the ZR and ER operate on 1550nm SMF), but they've published some of the optical parameters for the ZR XENPAK here.
EDIT: While I thought that 10GBASE-ZR was Cisco-specific, it's not actually. The wikipedia page for the 10Gig standard states "multiple manufacturers" have introduced "ZR" optics which are based on the 80km PHY described in the OC192 SONET specs.
With LX optics, you can connect two units with a short patch cord and expect them to work fine.
With ZX optics, you will need an attenuator if you are not connected to a long section of fiber. If the optical receiver is expecting a signal that has passed through 40-70Km of fiber, it can be overloaded by one that's passed through 5m of patch cord instead.
Looking at Cisco's specs, and remembering that every 3dB is double/half the power, we see that a receiver twice as sensitive is coupled to a more powerful laser (almost 4 times as powerful at maximum):
Note (under fiber loss budgets) that the maximum power out of a ZX is 5 dBm (min 0 dBm), and the maximum power in is -3 dBm - while the maximum power out of an LH/LX is -3 dbm (min -9.5 dBm), and the maximum power in is also -3dbm. That means a ZX with a low-loss link will overload its receiver, while an LX will not and cannot.
The minimum receive power is -20 dBm LX, -23 dBm ZX.
Thus, the total loss budget for a ZX is between 23 & 28 dBm, and for an LX it's between 10.5 & 17 dBm.
I don't necessarily agree with the previous answers. It is true that these longer range optics aren't part of the standard. But Cisco special? Maybe they first introduced them, I don't know. But these are quite commonplace.
The major difference is not the transmit power, but rather the sensitivity of the receiving side.
They are a lot more expensive, maybe that's the main difference!
If you aren't running DWDM/optical amplifiers, and have a one off long distance run or two, these could be a good fit for you. I am running some and they work fine.