My ratings are of course subjective, though I try to stay open-minded and considerate of individual circumstances. The four areas I rate are:

Ease to arrive

This is not a question of distance from home to destination; it’s about the ability to find the place. It’s fine if it’s a walk – but do you know which way you’re walking? Are the maps placed in the subway station correct? What about the directions in official publications? Five globes in this category means a place is easy to get to, easy to find the entrance, and easy to navigate the trip there; one globe means the place is very difficult to find without a lot of help.

Foreigner-friendly

I don’t want to be patronized; by foreigner-friendly I’m referring to understanding the place and feeling welcome. Are there English-language brochures or signs, or are the only explanations in the local language? Does the place seem welcoming to foreign tourists, or do they sneer at them? Another consideration is if there are different prices for locals and tourists. Five globes in this category means a place has plenty of information and/or people are very friendly; one globe means there’s little to nothing for the tourist to understand or learn, or something’s afoot to make it less enjoyable for tourists.

Convenience facilities

Walking around and exploring can get thirsty or tiring; thankfully, most places offer restrooms, convenience stores, rest areas, and the like. This rating asks if what you need to enjoy this place is readily available. Are the facilities clean, well-stocked, and reasonably priced? I certainly don’t expect every modern-day accessory at the top of a rugged mountain trail, but a bottle of water at the bottom of the hill isn’t too much to ask. Five globes in this category means you’re never too far away from what you need; one globe means you’re left wanting for everything. (Do note, of course, that many off-the-beaten-path or lesser traveled places lack basic facilities – this is factored in to help you prepare for the trip.)

Worth the visit

Some places will come highly recommended despite its remoteness, while others are not worth the time and effort even if it’s in the middle of town. Elements considered in this category include interesting things to see, do, or experience, the uniqueness of the place, and so on. Five globes in this category means it’s an exceptional place that merits a special trip; one globe means it’s barely worth mentioning except as a cautionary tale.
On posts written since mid-2013, I’ve used the One Weird Globe as the unit of choice:
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For some destinations in Thailand (posts from a portion of 2013), I used a ratchaphruek (pronounced rah-cha-proo-ek):
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Beyond being the national flower of Thailand, yellow is the traditional color of royalty, so naturally there’s a bit of prestige to it. The name translates to ‘Golden Shower’ (stop snickering) and its other name, chaiyaphruek, means a tree of triumph. The plant can grow to be over 10 meters tall, and each flower has five petals.
For destinations in Korea (posts from 2008 to 2013), I used a taeguk (technically the sam-taeguk variation):

From the concept of Taiji comes yin and yang – a swirl of blue (representing earth) and red (for heaven). This symbol is in South Korea’s flag, through a third swirl of yellow (representing man) is added as a variation. That’s the variation you’ll see on some palaces around Korea.