From reading these posts some common elements appear...
1. Allowing yourself to be emotionally vulnerable
2. Interacting with reciprocity; giving and receiving
3. Being authentic
Intimacy sounds a lot like a process, where people are brought from outside emotional walls into the realm of your authentic self through reciprocal interactions of giving and receiving.
@Kgal :
Your negative rep ("Not applicable to topic") on my above post - care to explain how it's not applicable? I mean, nothing says intimacy like unspoken communication.