Expensive little pointy bits
— What is the difference between Moving Coil (MC) and Moving Magnet (MM) cartridges?
— All magnetic pickup cartridges make use of Faraday's Law of Induction. This law describes how a change in magnetic field (flux) will produce an electromotive force (EMF) in the electric circuit.

There are two ways to exploit this natural fact. Either make the coils stationary and move a magnetic field near them. Or make a magnet stay in place, and move the coils in its field. We have just described the moving magnet (fixed coil) and the moving coil principles.

Moving Magnet (MM) cartridge

Because of various limitations, moving magnet usually means having a tiny little speck of a magnet on a cantilever moving near relatively huge coils (typical inductance is between 0.3 and 1 Henry). And in the other case, an almost microscopic coil moving in a stationary very strong magnetic field (yellow lines on the illustrations stand for the magnetic field).

Moving Coil (MC) cartridge
Typically, the electrical energy generated by MM and MC cartridges is comparable, but it manifests itself differently. MMs are capable of generating relatively large voltage swings that are easy to amplify (almost a volt peak to peak). MCs usually generate tiny voltages, but can produce more current. To effectively use the signal from the moving coil cartridge, you need a device that can convert current into more voltage—a transformer or a transimpedance (current-to-voltage) amplifier.
Voltage mode MC "head amps" usually sound disappointing because they only attempt to amplify the feeble voltage, ignoring the current.

— What's the difference between MI and MM cartridges? Do they have different sonic signatures? Is MI better?
— If we are talking about modern cartridges, sonically there is no clear advantage to, or clear distinction between MM and MI. "Recently" (1980-) designed MM and MI cartridges tend to sound very similar. But it wasn't always like that, and through many decades MI held a significant advantage.
Moving Iron (MI, fluxvalve, moving flux etc) cartridges appeared in 1930s. The permanent magnets available then were so weak that building a moving magnet cartridge was impracticable. Pickup designers had to use a stationary permanent magnet: either move a coil through its field, or move a little piece of iron to modulate the flux and measure the result with a stationary coil. The latter approach produced a lot more voltage.

Moving Iron (MI) cartridge (also called fluxvalve)
After the WWII, the science of permanent magnets developed by leaps and bounds. Magnets of the same mass became a lot stronger, and by the mid-1960s, moving magnet cartridges became a reality. They were also a neat way around General Electric's patents that prevented other companies from building MI cartridges. Those magnets were still relatively heavy, but a) clever engineers at companies like Shure, Audio Technica and ELAC found ways to minimize the moving mass and b) GE rested on its laurels, largely missing the stereo revolution and allowing MM cartridges to gain market share.
In the late 1970s, rare earth magnets were perfected. Samarium-cobalt magnet could finally be made tiny (and light) enough to match moving iron cartridges' moving mass. Neodymium magnets appeared in the 1980s. These super strong magnets negated the moving mass advantage of the MI configuration. Once rare earth magnets found their way into mass produced MM cartridges, MM and MI cartridges became similar in performance and sonic signature.

MI configuration has other potential advantages, but they remain largely untapped. Most of the research and development investment goes to improving the moving coil cartridges. Moving Iron configuration has its champions (Grado, Soundsmith, Nagaoka), but they haven't created a truly original new generator design in decades.

— Is the cartridge with the lowest moving mass always better?
— Not really. Moving mass is certainly an important factor, but there seems to be a threshold below which it doesn't matter so much any more.

There are plenty of cartridges with low and very low moving mass that do not sound special. On the other hand, there are amazing pickup cartridges with positively huge moving mass, ranging from Denon DL-103 to Audio Technica AT-ART1000.

— Are MC (moving coil) cartridges always better than all other types?
— Generally, most very high performance magnetic cartridges are indeed of the moving coil type.

MC's position at the top of audiophile pickup hierarchy is due to a combination of factors. They are listed below, in the order of relative importance.

  • Low inductance, and thus low energy storage. MC coil inductance is 2-3 orders of magnitude lower than MM or MI's
  • High body rigidity: MC cartridges are typically milled or 3D printed out of metal
  • A nice and clear vibration path away from the stylus. The suspension is usually directly connected to the cartridge's body
  • Reasonable moving mass: even when wound over an iron core, the coils do not weigh much
  • Stable constant magnetic field around the coils

It's interesting to note that nothing really prevents anyone from building an MM or MI cartridge with the same attributes. But cartridge makers prefer to focus their engineering investment on moving coil ones.

Exceptions like the Top Wing prove that the cartridge mustn't necessarily be of moving coil type to achieve remarkable performance.
There is an oft-repeated misconception that MCs sound better just because of their lower moving mass. The lowest published moving mass cartridge is a moving magnet (MM), Technics EPC-100C Mk4.
Please note we are only discussing magnetic pickup cartridges here. There are many other types, like optical, strain gauge, capacitive etc. All of them have remarkable sonic potential and can be built to a level competitive with top MCs.

— How long does the stylus last? How do I know it's time to change it?
— Anywhere from 500 to 3000 hours. See the next question for the factors affecting longevity.

The second part of the question is even harder to answer. Without an advanced microscope, it's next to impossible to tell when the stylus is due for replacement. You might "feel" that something is off, but sometimes the worn styli even measure quite close to new ones.

If you tend to use only one cartridge/stylus, calculating replacement interval is a good idea. Listening on average for an hour a day, you should be buying a new stylus in 4-5 years' time.
Put a little sticker on the back of your turntable with the date of stylus replacement, and you will know for sure when to get a new one.

— Is it true that advanced styli last longer than simple ones?
— All other things being equal, yes, to an extent.

Many factors influence the stylus longevity. The geometric shape of the stylus is, indeed, one of them. Simple conical styli wear fastest. Ellipticals and hyperellipticals seems to be the optimal shapes for longevity. Some more advanced line contact styli last less, and sometimes (thank God infrequently) fail catastrophically, with bits chipping off.

But the quality of the diamond itself is arguably more important than the shape. A polished grain-oriented conical, like in a Denon DL-103, will easily outlast a bonded hyperelliptical. Natural mined diamonds used in the 1970s and 80s seem to last a lot longer than today's synthetics.

— Do styli have expiry date? Do cartridges?
— Styli and cartridges with integrated (non-removable) styli, yes. Cartridge bodies without a stylus, generally no.

The shelf life of the stylus is determined by the elastomer "donut" holding the cantilever in place. These elastomers vary widely in longevity. Some keep for decades, some for just a few years. Generally the softer they are the quicker they go wrong, but there are exceptions.
When buying styli and, especially, moving coil cartridges, make sure you are getting a recently made one. There are more than a few unscrupulous retailers selling styli produced in the 1980s (and even 1970s!) as "brand new".
MM/MI cartridge bodies are just a few coils in a box. Nothing to go wrong there... usually. In reality, copper can corrode, cores can delaminate, and the glue holding the cartridge parts together can turn into dust. This happens very seldom—typically when you receive that elusive cartridge body you were long searching for, shipped at a great cost from across the globe.