Unicode contains a wide range of different scripts.

For example, some are phonographic (the letters represent phonemes, i.e. speech sounds), others are logographic (the letters represent words, or morphemes).

Both of these can be featural: The Korean Hangul script is phonographic, but the letter shapes are not arbitrary. Some of them are visualizations of mouth shape or tongue position when producing the sound. For example the velar consonant ㄱ (somewhere inbetween the English g and k, roughly) is the tongue position as seen from the side.

A logogram can also be featural, i.e. it can look like the thing it represents. This would be the case for a glyph that means "thermometer" and looks to viewers like a thermometer. It could also be arbitrary - some shape that you just agree on to mean thermometer. Perhaps sharing some graphical aspects with topically related logograms.

They're all just spots on a spectrum.

(Edit: I briefly touched on pictograms vs. ideograms a bit more down below.)

That's amusing, because I looked at the thermometer, and thought, oh that's good, I can use that. But a chilli pepper, do we really need a character for a chilli pepper? So to answer your question, unicode is trying to bridge the gap in communication that offers a language independent set of recognizable glyphs that can be used to communicate. And one mans chilli pepper is another mans thermometer.

I suppose that the chilli pepper is there for restaurant menus which often use it as an indicator of spiciness.

Is the thermometer symbol used widely? No idea. Is emoji used widely? Yes, In Japan 🇯🇵 (and Korea? 🇰🇷) and has been for > 15 years. A common email/msg composed by a Japanese woman 👧 age 15 to 35 uses many emoji.

I realize this message will not render correctly in Chrome because Chrome, at least Desktop Chrome, does not yet support emoji 😭

Is the problem with Chrome or the OS and font selection? I think everything renders on my Win 7 + FX 30 setup I'm using now. Generally, I have the best success with Win 8 + IE 11.

On OS X, Chrome renders boxes, Safari renders the proper characters.