What's the main differences between QCLs and ICLs in regard to laser characteristics?
The threshold current density is indeed lower in ICLs than in QCLs, mostly because of the large difference in carrier lifetimes (~ nanosecond in the ICL, ~ picosecond in the QCL). The lifetime is intrinsically short for the intersubband lasers because of the fast intersubband scattering (from phonons or interface roughness).
There are many other differences. Here are some of them: (1) the QCLs are much less temperature sensitive; (2) the QCLs use more stages and can generate higher output powers from a single ridge; (3) the QCLs are unsuitable for vertical cavities because of low gain for in-plane-polarized light, whereas IC VCSELs have been recently demonstrated etc.
The active regions of ICLs are typically type-II quantum wells, in which their bandgaps are determined mainly by the quantum well layer thickness rather than the bandgaps of the constituent materials. Hence, the bandgap of an ICL can be tailored to cover the entire mid-infrared spectrum.