What are important design parameters for Opto-isolators?
The single most important parameter that's peculiar to Opto-isolators is CTR (Current Transfer Ratio). It's kind of like \$\beta\$ for a BJT, in that it defines how much current shows up on the output (photo-diode or photo-transistor) side given an amount of current driven into the LED side. CTR is a function of temperature (positive temp coef) and age. CTR falls off by about 30% as the part ages. 

The 6n135 is a fast(ish) part where the optical transfer is from LED to photo-diode. That means that for the part to work the cathode of the photo-diode (Vcc) has to be connected to a bias voltage, typically 5V. Photo-diode current gets buffered through a BJT output transistor, which adds some current gain to the CTR number. If you look at Figure 4 (page 4) of the data sheet you will see how LED current converts to output collector current.  

For more about this you could look at App Note 02. The first 2 pages talk about CTR. Also good is App Note 45.