Highly radioactive usually has more to do with faster decay rate. As for how dangerous, it depends on the emitter and how it is absorbed. As for how much energy, it depends on substance, if it is fissile (at least for energy producing), and its neutron efficiency. Thorium, uranium, and plutonium generate more neutrons than they consume and thus can be used for a sustainable nuclear reaction. If it isn't one of those three, it probably isn't desirable - Protactinium, for example, has a huge cross section and absorbs neutrons slowing the reaction, so in a reactor it is usually desirable to pull it out, wait for it to decay to Uranium, and toss it back in (but this is a proliferation concern :P ).
Alpha - Ok for skin exposure, bad in stomach, lungs, or other tissues
Beta - relatively OK for skin exposure, bad in stomach lungs or other tissues (but not as bad as certain alpha emitters, I believe)
Gamma - pass through organics, bad for them.
For instance, Polonium is a fast alpha emitter. Skin is very good at protecting against alpha emitters, so you could wear gloves and handle it (to avoid any chance of dermal absorption). You, however, in no way want to ingest it - in the lungs and tissues it wreaks havoc and can kill in days (which is why Polonium was used to kill Alexander Litvinenko, a Russian dissident). Beta emitters are mostly absorbed by the skin, but penetrate deeper than alpha emitters. Gamma emitters go through most everything except heavy metals like lead, so it is recommended that you get as little exposure to these as possible (either inside you or outside).