Nicodemus,
My belief is that one should avoid mixes that contain perlite, vermiculite, slow-release fertilizers, etc. Perlite is that white ball-shaped (pea sized and smaller) stuff that crumbles when pinched. Tortoises are naturally inclined to eat white stuff, maybe they assume it's calcium? Vermiculite is that off-white, layered stuff, similiar to perlite, except it compresses when pinched. Both add drainage to mixes. Also avoid mixes that contain those slow release (round) fertilizers, e.g. osmocote, and definitely anything with insecticides. Essentially, you need to avoid anything that could be ingested and cause compaction. Many keepers recommend against using anything but "reptile safe" barks, etc.
Most potting mixes are made from peat moss and therefore I use a combination of sifted peat moss (remove sticks, etc), washed play sand, and a heafty dose of limestone (calcium carbonate). The limestone increases the pH of the mix (peat is acidic) and the calcium is also absorbed by the plants, which then get eaten, etc. This mix is basically sterile (unlike compost-based ones), can hold moisture (great for redfoots), high in calcium, and best of all *cheap*. If planted, you will have to water it occasionally with a fertilizer solution, just be sure your tortoise doesn't drink any. I've raised a Herman's tortoise on this mix for two years now without any problems. He enjoys digging around in it two.
The only drawback for me is that the peatmoss can be dusty when dry. The sand reduces this, and I typically will wet the mix before scooping it out to keep the dust down.
Hope that helps,
Jeff