I was not surprised that the International Astronomical Union decided that Pluto isn't a planet. Even immediately following its discovery, there were doubts about assigning it that status. It's too small, the orbit is too eccentric, and so forth. But the IAU's actual decision to call Pluto a "dwarf planet" just doesn't sit well with me. (After all, it's not named Dopey.)
In astronomy, some breakpoints are easy to define. For example, the difference between a gas giant, a brown dwarf and a star is that gas giants are too small to achieve deuterium (double-mass hydrogen) fusion, brown dwarfs _only_ achieve deuterium fusion (which doesn't last long, since that form of hydrogen is rare) and not regular hydrogen fusion, and true stars are big enough to achieve the fusion of ordinary hydrogen atoms.
The breakpoint between gas giant and rocky planet is more obscure. The best determining factor is that a true gas giant must have most of its mass in elements and compounds which are gases under terrestrial surface conditions. That firmly places Venus in the rocky planet category. Though having far more atmosphere than Earth (the majority of the components which comprise it are generally gases under terrestrial conditions) the total mass of atmosphere is still far less than the mass of the rocky portion.
Part of the problem with determining exactly what Pluto is comes from the fact that we don't know exactly what it is made of. We only recently launched a probe to perform this task. (The fact that we could have had a probe do this _within Clyde Tombaugh's lifetime_ and _didn't_ is scandalous.) Currently it is thought to be mostly rock with a thick layer of ice and frozen gases (again, for "gases" read "materials which are gases under terrestrial surface conditions") on the surface. We have pretty good estimates of its physical size from telescope observations, and the periods of its moons gives us a direct measure of the mass. That lets us calculate the density, which can leads to believing the mix listed above is accurate. So Pluto is basically a very large asteroid which is far enough from the Sun that its primordial ice and gas haven't completely evaporated away. That really doesn't fit the image of either one of the rocky inner planets, or the gas giant outer planets.
Then there's the orbit. I heard a newscritter who was talking about this matter smugly announce "Of course Pluto isn't a planet. Planets have cir-cu-lar orbits. Pluto has an e-lip-ti-cal orbit." Ignoring all astronomy from Kepler on and implying that the scientists who hadn't used this distinction to settle the matter were all idiots.
Now, I can see where she got this misconception. All planets have elliptical orbits, but for most the difference from a true circle is small. So she ignored that variation, mentally rounding both the orbits and the math. And also ignoring that this small difference drove many astronomers to distraction for centuries until Kepler finally figured it out. Pluto, on the other hand, has an orbit which is obviously elliptical. So elliptical that it crosses the orbit of Neptune. One theory - which also explains some of Uranus's peculiarities - is that Pluto was either previously a moon of Uranus or a moon of whatever knocked it over on its side.
That orbit is a major blow against Pluto being a planet. Because planets shouldn't cross the orbits of other planets. Add in the steep, 17 degree inclination from the plane of the ecliptic - several times that of any true planet - and you have something which looks tacked on and doesn't really seem to belong with either the classical planets or Uranus and Neptune. Of course, the major asteroids don't cross the orbits of planets, either, so I don't think Pluto should be counted as an asteroid.
But "dwarf planet"?! No, thank you. Don't you people have any romance in your souls? Not that I object to the classification _per se_; as Pluto did after it, Ceres confounded the orderly notions of astronomers at the time of its discovery. However, "dwarf planet" and "minor planet" have long been used to describe the largest asteroids.
Personally, I think we should declare Pluto to be the first-discovered of a new class of bodies. "Kuiper Belt Objects" would work technically, but most of what is out there would be comets, with far a far higher percentage of gas than is possessed by Pluto. Also, as mentioned above, Pluto may not be from there. So my suggestion is to declare Pluto to be a "Tombaugh Object." And use the opportunity when someone asks "A _what_ object?!" to explain about the modest farmboy who discovered something new in our universe. A very American story which the members of the IAU who changed what Pluto is obviously don't appreciate.