7200RPM WD Blacks are designed to be faster then WD Blues/Greens.
They are the same speed, same cache, etc., what really makes them actually faster?
Does it have to do with spin up/down speeds or what?
The list of differences I managed to compile while shopping for xmas was:
Other factors to consider include interface speed, platter density, speed of the cache, and the design of the drive controller.
Blacks also have support for various commands (e.g TLER) that are important for enterprise users (RAID storage, etc) but generally not useful to the home user.
WDC have a list of the features unique to Black on their site.
To answer the first part:
Nothing. There is nothing special about 7200 RPM drives.
They are often faster than their lower RPM drives because most mechanical hard drives store their information on a rotating platter. Spin the platter faster and it passes faster between the point where the information is read or written.
If you take a platter from a 7200 RPM drive, slow the drive down to 5400 and re-low level the drive then you will end up with a drive reading and writing at ¾ of the 7200 speed drives.
(5400/7200 = 3/4, or ¾).
It will also have a slower random access time since to access a piece of random data you need to wait an average of half a platter rotation. If the platter moves faster then this takes less time.
Is 7200 RPM special?
No, drives have been produced with all kinds of rotation speeds, platter densities, number of platters etc etc. It is just that lots of the recent drives are either 5400 RPM or 7200 RPM. Those 7200 RPM's often get other beneficial features while the 5400 RPM often get aimed at low power, low noise environments.
That is not to say that those are the only speeds. Drives I have used in the last 6 years included rotation speeds of 4200 RPM (bigfoot), 5400 RPM (original from my laptop), 5900 RPM (my newest 4TB drive) and 15k RPM (a small SAS drive in my desktop).
Higher than 15K RPM does not seem to be used. Probably because the added effort to does not warrant the performance gained.
Lower than 5400 RPM is also rare. Usually only in ultra-low-power drives or in drives with large platters (e.g. the bigfoot series which was the last 5 ¼ consumer drive I saw).
Still, even if it is used a lot it is nothing special.
One advertised difference is "dual processors".
All HDD designs are a balance of audio qualities, power saving plans, disk head movement optimizations, cost and so on. I've found Black drives to be fairly noisy, so I would guess the optimizations are slanted towards performance over audio qualities.