The initial definition of the word "moe" is far different from the one we use today. Originally, it referred to an interest in a particular kind of character; for example, a catgirl fetish would be "nekomimi-moe," a glasses fetish would be "meganekko-moe," and so on. Over time, the term evolved to describe any anime or manga character that elicited feelings of protectiveness in male viewers. Unfortunately, as we've already seen, that definition is far too broad to be practical. Therefore, based on common archetypes present in the recent 'moe boom', I've compiled the following Four Laws of Moe, with a little help from my favorite moeblob Ritsu Tainaka. I believe that any character who adheres to three of these four laws can be considered moe.

Moe characters are cute.

The concept of 'cuteness' is very important in Japanese pop culture, and moe exemplifies this. A moe character will always be as cute as possible, even at the expense of being sexually attractive. Naturally, this lends to their merchandising potential; hardcore otaku are always ready to buy figures and posters of their waifu. In order to facilitate maximum cuteness, they will almost always be female and adhere to laws two through four.

Moe characters are young.

Generally, moe girls will be around high-school age, the time when they are just starting to explore their sexual identity but still retain childlike innocence. This allows them to be fetishized without making the viewer feel uncomfortable about lolicon, but still preserves the cute naivete that creates feelings of protectiveness. This protective instinct is often linked to feelings of fatherhood, especially among male otaku in their twenties and thirties. Incidentally, this demographic also has a great deal of liquid capital to spend on the aforementioned merchandise.

Moe characters are innocent.

This goes hand-in-hand with the second law. Moe girls are almost always virgins, and frequently have had no romantic experiences whatsoever. They often have a cheerfully optimistic view of reality, and are completely unaware of the feelings and/or perversions of those around them. In their first romantic encounters, they are usually extremely shy and tend to blush a lot. This makes them seem both physically and emotionally vulnerable, once again adding to the cuteness factor and creating feelings of protectiveness.

2chan's infamous 'virgin hunters' value this trait above all else... even going so far as to throw temper tantrums when they discover their waifu isn't as uncorrupted as they had thought.

Moe characters are quirky.

Moe characters usually have some sort of quirk that makes them stand out from the crowd. This can be a unique physical feature (i.e. lolfang or twintails) or an unusual way of speaking (i.e. desu or nyoro~n). Another common quirk is behavioral in nature; common examples include the lazy girl who is always late or the tsundere girl who inflicts pain on her crush. The best kind of quirk adds to the overall cute factor of the moe character in question, without being socially undesirable. For example, running to school with toast hanging out of your mouth is fine... but collecting porn magazines is not.

As a test, let's apply these laws to a few examples from an old favorite of mine, The Melancholy of Haruhi Suzumiya.

Kyon

Haruhi Suzumiya

Yuki Nagato

Mikuru Asahina

And that's my proposed system for determining moe. What do you guys think?

Tl;DR: Go back and read the whole thing, you lazy jackass!