Newbie Question: MM v. MC cartridges?

Generally speaking, yes you're missing out. Most of the top carts are MC, although there are certainly some nice MM out there. The ear phono stage is a good one, and with a nice table, you will certainly hear a difference.
Check the archives and you'll see lots of discussion over cartridge loading. You need to make sure that any cart you buy is a good match with your phono stage. Alternatively, you could buy a step-up transformer which will help with lower output cartridges.
The FAQ here and on audioasylum's analog forum will be helpful. Cheers,
Spencer

I wouldn't say that most pickups are MC these days--there are still a fine selection of each, in widely varying price ranges. Clearaudio makes excellent MMs, as do Grado, Dynavector, and a number of others.

On the other hand, I could conjecture that MCs have been gaining in popularity with the wide disappearance of typical MM stages in low-end gear. Vinyl has gotten to be more "high-end" oriented, and in the high-end camp you're much mroe likely to find enthusiasts who own the high gain stages necessary for the low output of fine MC cartridges.

I'll let people with more experience talk about sonic difference, but I will say that in my limited experience, I have heard good MMs sound very similar to MCs...

There are quite a few MM cartridges out there. In addition to those cited by Dirtyraggamuffin, Goldring, Garrott Brothers, Musicmaker, Roksan, Audio Technica & Ortofon make some very nice & very price-worthy MM cartridges.

W.r.t. MC cartridges: Depends on what you want out of your system. MC cartridges, in general, are about a more refined sound. The construction of the MC cart. allows this vs. a MM cart. Tradtional MC cartridges are low output & this is done primarily to make it a better groove tracker, which is how it gets its better ability to read the groove info.

As audiophiles progress in this hobby their hearing tends to get refined, in general, & they tend to congregate towards MC cartridges. However, some of the newer Grado Statement 0.5mV MM cartridges are very refined MM cartridges! If you heard one you'd know what I'm talking about.

As you well know, refined sound costs money hence the MC cartridges tend to be expensive. It makes sense to use a MC cart. if your TT system is resolving enough that it will allow the transporting the wealth of info picked up the cart. to the phono preamp. Further, the electronics must also be of fairly high caliber to allow this info to get thru the speakers & to your ear. Needless to say the speakers must be very good too. Otherwise, you end up w/ mediocre sound & you ask yourself if all that expense was worth it! An explanation is very general terms, IMHO.