Why do they consider radioactive matter with long half lives more dangerous than matter with a short half life? The title says it all.
For example why is plutonium considered more dangerous than radioactive iodine?
 A: Half  life is defined  as

the period of time it takes for a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms (radioactive decay), but it may apply to any quantity which follows a set-rate decay.

The shorter the life time the faster the material returns to normal levels of radioactivity.
Iodine in eight days exudes  half the radiation it had the first day. In sixteen days it is 1/(2*2)=1/4 etc.
Plutonium takes hundreds of years to get to halve its radiation level.
In addition the danger of the body replacing its minerals with radioactive ones is a major factor of danger from long lived elements, like plutonium, through the food chain, from fallout and ground pollution. The shorter the lifetime , the better.
A: A more balanced approach might be to recognize that both short and long half-live materials can be serious hazards, but usually for somewhat different reasons. Also, the devil is very much in the details here, because issues such as how your body absorbs the isotopes is also very, very important.
Radioisotopes with short half-lives are dangerous for the straightforward reason that they can dose you very heavily (and fatally) in a short time. Such isotopes have been the main causes of radiation poisoning and death after above-ground explosions of nuclear weapons.
Iodine is an example where preferential absorption by the human body can further aggravate the dangers of short-lived isotopes.
Long-term isotopes are more complicated. They don't dose as heavily, but there are a lot more issues than just that. Plutonium for example is comparatively long-lived, but some of its decay products can be quite nasty. Also, plutonium happens to be particularly toxic due to its chemistry, which aggravates the damage it can do.
The biggest danger from radioisotopes with mid-to-long half lives is that they can keep an entire region of earth nastily radioactive for a very long time, e.g. hundreds or thousands or even tens of thousand of years. That's the main reason why disposing of reactor wastes, which often contain just such isotopes, is such a contentious issue.
At the extreme end are isotopes that are so long-lived that their hazard levels are close to zero. Uranium-238, the kind left after the fissile 235 is removed, pretty well falls into this category. Bismuth (as in the main ingredient in a popular pink stomach relief aid) is ironically in this category, with a half-life so long it's hard even to tell that it is radioactive.
A: 
why is plutonium considered more dangerous than radioactive iodine?

Because the press have heard of Plutonium and Pu=atomic bombs=bad
Plutonium's danger is over stated, it's insoluble so hard to get into the food chain and even if ingested is going to go straight through you. Pu is only a real concern if breathed into the lungs as a fine dust.
Iodine is much more of a concern to human (and animal) health it is readily absorbed in the body and is sufficiently active to have serious radiological effects.  The only good thing about Iodine is that if it occurs in a reactor accident on the other side of the world and takes 8days to get to you - there is only half as much of it left.
A: There are two aspects to be considered for radioactive material. Long half-lives like Pu will contaminate areas for decades or centuries but have relatively small decays/second per unit mass. Short half-lives  like Iodine disappear in months but have extremely high decays/second per unit mass. Of course, ingesting either can lead to serious illness or death.
A: It might have to do with metrics. If Becquerels are used, 1 Becquerel equals one distintegration per second, then one becquerel of a long halflife substance means there are more radioactive atoms present (by the ratio of halflives). It all depends on how you normalize things. If you normalize by the number of radioactive atoms present, then the shorter halflife material will have many more disintegrations per second initially. If you normalize by instantaneous activity, then the shorter half-lived substance will always jave fewer distintegrations per second. As usual, some actual numerical understanding, as opposed to rhetorical understanding must be applied.
In terms of contamination, contamination by short lived stuff, can probably be waited out. For instance if milk has too much I-131 (halflife 8days), and it were made into powdered milk or cheese, and left on the shelf for a few months it would then be harmless, whereas if a foodstuff were contaminated with Cesium-137 halflife 30years, it is impractical to wait for it to decay.
A: Radioactive iodine is used for diagnosis of thyroid diseases such as hyperthyroidism and hypothyroidism. In such cases very little radioactivity is orally administered. Since it has half life of 8.0207 days, it is considered safe to use for diagnostic purposes. Moreover, it is excreted out fast through urine. Iodine-131 is orally administered in higher radioactivity levels to treat hyperthyroidism and thyroid cancer. In comparison, if a long lived radioisotope like 137-Cs with 30.07 years half life goes inside the body in situations like Chernobyl reactor accident then certain portions of body receive more radiation exposure. Excessive exposures can be unsafe.  
A: Scientists consider radionuclides with short half lives to be a lot more dangerous because they are.
Press weenies and environmentalists have a political agenda and spread the lie that long half lives are bad.
Uranium and Plutonium are hardly a health hazard at all, at least in metalic form, and certainly not a radiation hazard unless you happen to get a critical mass together).
Of course in dust form, particularly as salts or oxides, they are highly toxic, but that's not a radiation hazard but a chemical one.
Short lived isotopes cause the high radiation environment you don't want to be exposed to for prolonged periods. Those environments of course also don't last very long as decay will cause those isotopes to disappear after short period (days to decades).
This is plainly visible in the area around Chernobyl today where background radiation (except in some hotspots) has fallen to a level where it's been safe for people to work right next to reactor 4 without special protective clothing for decades, where now the Ukraine is actively promoting tourism into the area which is a haven for wildlife.
It's also no surprise that both Hiroshima and Nagasaki are not wastelands but bustling cities where people live with no fear of radiation sickness and have since shortly after WW2.
This of course doesn't stroke with the alarmist message that "the site will remain dangerously radioactive for a hundred thousand years".
The danger from short lived radioisotopes is twofold:
1) direct radiation exposure from decay is high and intense, which can cause radiation sickness, cancer, etc.
2) many are readily absorbed into the human body, and when decaying after being used as building blocks for cells (especially DNA) can lead to mutations, cancer, and other health problems.
While theoretically possible for some long lived radioisotopes as well, absorbing into your tissue some of those won't likely lead to decay related problems during your lifetime as there simply won't be enough decay during your lifetime for there to develop problems larger than your body can correct.
