First off, the things we put into space, even the things we use to put people and other things into space, are not really friendly when you get right up close. Keep in mind that in order for the Space Shuttles to be decommissioned and turned into aerospace museum exhibits, they had to go through an extensive decontamination process to remove all the volatile, toxic chemicals used as propellants, fuels and even lubricants on the orbiter, replacing them with substances that only have to do the job well in Earth's atmosphere and ambient temperatures, instead of at +-200*C in a vacuum.
One example, a common fuel used for lift, deorbit and station-keeping/orbit-modification rockets, is hydrazine, N2H4. Ideally, one molecule of hydrazine and one molecule of diatomic oxygen combine to produce two molecules of water vapor, one molecule of diatomic nitrogen, and about 592 kJ/mol. For the size of the molecules involved, that is a lot of heat released, which is why it's used in rocket fuel; pound for pound, it's better than the well-known explosive C4 (C4's primary active ingredient, RDX, gives you a little over twice as much energy per mole at about 1120 kJ/mol, but weighs almost 4 times as much at 222g/mol to the hydrazine and oxygen's combined 64g/mol). With a rocket being little more than a long, controlled explosion, the rocket's fuel tank is not something I'd like to see entering the oxygen-rich atmosphere at hypersonic speeds; that's a one-way ticket to a short, uncontrolled explosion, and a powerful one (especially with the LOX tank entering right beside it bringing lots of concentrated oxygen goodness to the party).
Hydrazine is also acutely toxic; the NFPA, which rates chemical hazards for use in industrial settings, gives it the maximum rating of 4 as a health hazard, placing it in the same category as substances we have developed specifically to kill each other, like hydrogen cyanide, sarin, and VX nerve gas. The substance will also spontaneously ignite at ambient Earth temperatures and the presence of oxygen (another reason it's used as fuel; no ignition source needed), and it will go ahead and spontaneously detonate given a good physical shock (like, say, the impact with the ground after re-entry if it makes it that far). With this overall NFPA 704 rating of 4-4-3, it's among the nastiest substances our chemical industry produces in bulk (there are nastier, but they're typically not sold and shipped by the tanker-load).
Lastly, the ideal reaction between hydrazine and oxygen isn't the only possibility; given various mixtures of the two, you'll end up with ammonia gas, various oxides of nitrogen, and other not-nice things (though almost anything's better than hydrazine itself). And that is if LOX is used as the oxidizer; there are things known to modern science that oxidize better than liquid oxygen, like the halo-trihalides (chlorine trifluoride, chlorine tribromide and bromine trifluoride), perchlorates, peroxides etc, all of which could be in the satellite's oxidizer tank and would combine with hydrazine in new and exciting ways. A common one is nitric acid (in its white fuming form, often "inhibited" by adding hydrofluoric acid which creates a protective layer of metal fluoride, to prevent the nitric acid eating through the tank), because it doesn't require cryogenic handling; the ideal reaction would be one part fuel to two parts oxidizer producing nitrogen gas, water and oxygen, but nitric acid doesn't like to act "ideally", and you're much more likely to get a (un)healthy dose of toxic nitrogen dioxide instead of the clean nitrogen and oxygen.
This is all just from one example, with "interesting" consequences from a chemistry perspective. More glaring is the simple physics; a spacecraft in geostationary orbit is travelling at about 11,068km/h, or about 3,074 m/s. It's an interesting contradiction in projectile physics and orbital motion, that "you speed up to slow down". The opposite holds; by slowing this spacecraft's angular velocity down to de-orbit, you end up going faster relative to the earth's surface. By the time you begin re-entry, you're actually going faster than 8,000m/s, or faster than Mach 25; 10 times faster than a .223 Remington round fired from an M-16. And the satellite weighs a hell of a lot more than a .223 slug.
Just as a rough estimate, let's say the spacecraft weighed a modest 5 tons (~22000 kg), which is the current average for a communications satellite. At escape (or reentry) velocity, the craft would have a kinetic energy of .5 * 22000 * (8000)^2 = 7.04E11 J. In perspective, that's a little more energy than you'd get by detonating 10 MOABs simultaneously. Adding back in the hydrazine goodness, let's say 400kg of the craft's weight was hydrazine fuel. At 32g/mol, and 592 kJ/mol, the potential chemical energy of the hydrazine fuel is 7.4E9 J, one one-hundredth the energy of the spacecraft itself, but still about the energy inherent in the average lightning bolt.
All figures from Wikipedia.