[bkcooper]

How does electric current through a (non-super-) conductor convert energy to heat?

You have a coupling to other, non-electron/hole degrees of freedom in the system. The stronger that coupling is, the easier it is to transfer energy between the systems and the worse your conductor tends to be.

[acqq]

As much as I know about the subject, I like this answer more than all others here. The "bumping" interpretations don't appear to match what we know about the atoms and the particles for many decades and look to me like an explanation for small kids.