What are the effects of single ended signals on twisted pair conductors?
Essentially, I'm only using one conductor of each twisted pair.


Actually, you're using 2 conductors. One conductor of the twisted pair, and another that you haven't specified, which returns the current, or is the reference voltage. It will be there, even if you don't know where it is at the moment.

Is a ground conductor in the cable, or a ground loop from your transmitting end to your receiving end? Or is it the other wire in the twisted pair.

If it's a ground conductor in the cable, and it's working OK to share that between several circuits without too much crosstalk, then that's OK. The other redundant conductor twisted with it sould not be too much of a problem either left open or paralleled with the signal conductor, as long as the frequency is fairly low.

If it's a random ground loop, then that's almost always bad. You need to know where your return is.

If it is the other conductor in the pair, then you will be using it at the far end as the reference for the signal, by taking both conductors into a differential receiver. This will isolate the signals from externals signals, and the pairs from each other. That's the best way to do it.