I'd guess that the degree to which the group attached at the 4 position is able to 'pull' the delocalized pi electrons of the aromatic ring will have a correlation to the potency. Obvious other factors (as mentioned above) are polarity and size. Polarity is important as any highly polar group will not fit into the space allowed by the receptor molecule, most probably because of electrostatic charge repulsion. Also it will severely hinder the ability of the molecule to cross the blood brain barrier. Size is a factor, as shown by Shulgin - the fact that the DOx compound with a pentyl group attached to the 4 position (DOAM) is inactive.
Attached is a jpeg showing the effect of the CF3 group on the delocalized pi electrons. All of the halogens (and some groups such as -CN and -CH2CH2F) have the same effect on said electrons, but to a lesser degree.
Possibly anything capable of doing this to a greater degree than CF3, but while remaining lipophillic (non-polar) would turn out to be even more potent (if anybody out there with a good understanding of mechanisms of org chemistry can suggest a better group, don't be shy!)