Oh one thing I meant to mention. Someone asked about "pop-a-top" Russian tanks a while back. I heard a good explanation for this (and a bit of interesting history).
If you look at Russian tanks (T-72 being the most popular, yes, that means it was first deployed in 1970!), they are comparatively small, especially the turrets. A small tank of course has its advantages, and a small turret very much does since in many cases, that is all you can see. The reason why they can make the turret so small (western turrets are much larger) is because they have three man crews (commander, driver, gunner) and only two in the turret. Wester tanks have a third crew member in the turret, the loader.
Russia gets away with this by using an auto-loader for the main gun. What is the downside of an auto-loader you ask? Well, a smaller crew means any maintenance is harder but... also means that the ammunition for the main gun needs to be stored in the turret (I think you know where I am going here). Of course, having the ammo stored inside the turret means most any penetrating turret hit results in lighting off the main gun ammo. Pressure builds up very quickly, pressure has to go somewhere... turret flies off!
As noted previously, many times the turret is all you see, and not a huge surprise here, many of the ATM's (e.g. Javelin) aim for the turret. Yes Demitri, that is ammo you are sitting on:
Interesting historical tidbit: You may have heard about how dominant German tanks where on the western front in WWII. You also may have heard how the US discovered that they could effectively deal with the German tanks if they had numbers. You might think that they would rush them and surround them and hit them in the soft spots, but apparently one of the most effective tactics was a bit different.
Imagine being in a tank, specifically the (steel) turret, and a tank round (14lb shell traveling 1300 mph), hits the turret, but does not penetrate. It would be a bit harsh to say the least. Well, bounce a number of shells off of a turret, and you have what you might call "ringing the bell". Most, less than super hardened tank crews, would want none of that and would tend to bail out.