There is a spectrum of attraction and affection, which of course exists in both English and Spanish. How one expresses their level of affection and attraction along the spectrum is a difficult thing to pin-point in any language, and often subject to interpretation, body language, and other clues. But generally speaking, I think it's fairly safe to say that Spanish is a bit more precise than English in this regard.
In general, the progression seems to be, in English:
- Me caes bien.
- Me gustas.
- Te quiero.
- Te amo.
So clearly, Spanish has a higher resolution in this regard, which makes translating difficult. But it needn't make understanding difficult. We compensate for this weakness of the English language by adding a lot of extra context. If we want to say "Me caes bien" in English, we do this by adding a lot of extra (and often clumsy) words, or by completely rephrasing, to get away from the ambiguous "I like you", and a possible romantic mis-interpretation:
- I like you, as a friend.
- You are a great friend.
- You are a great person.
- I enjoy spending time with you.
When we're more squarely in the "romantic" realm, a simple "I like you" is more common in English.
Then somewhere between "I like you" and "I love you" is a big grey area in English, which is largely covered in Spanish with "Te quiero." In English, the moral equivalent to "Te quiero" would be things like:
- Love ya!
- ILY. (Acronym for "I Love You", but very informal)
- I love you. -- But in contexts, situations, or said with a tone which indicates a playful/less serious intention.
And finally, "I love you", complete with all the commitment issues between partners, or as you might say to your mother, is "Te amo" in Spanish.
It's important to note that in both languages, it's acceptable and common to use the "lesser" phrases even between people who are more familiar. As an example, just because I love my mother, doesn't mean I couldn't tell my mother "Love ya!" as I'm walking out the door. And just because I might say "Te amo" to my wife doesn't mean I couldn't also say "Te quiero" or "Me gustas mucho", when the situation called for it.