I understand that after fusing lighter nuclei, a main sequence star will either stop fusing or the fusion does not release enough energy to continue the sequence.
What physical properties make iron different in this respect? I'd also like to understand the importance of stopping fusing versus energy release with respect to iron ending the main sequence.
Roughly speaking, fusing nuclei lighter than iron releases energy, but fusing nuclei heavier than iron absorbs energy instead. This turnabout has to do with the interplay going on inside the nucleus between the (very short-range) attractive force between protons and neutrons and the (long-range) electrostatic repulsive force between protons. Iron happens to occupy a special spot where those dueling forces produce an equilibrium that represents a maximum in the binding energy per nucleon, yielding a nucleus that neither wants to fission (because of electrostatic repulsion) nor to fuse (because of the attractive residual of the strong force between nucleons.
I'm not sure I understand the second part of your question, but note that the reason some stars fail to fuse their fuel all the way to iron is because they aren't heavy enough to create the core pressure and temperature needed to do the trick, and there is a broad range of stellar masses represented in the main sequence.
