I think nice is a really boring descriptive word. I'm fun, and I think funny, helpful, friendly, smiley, compassionate, positive and understanding, and all these add up, I suppose, to me being nice. But, I'll also happily defend my boundaries, complain (politely) about anything I'm unhappy about, I can be quite cheeky, so someone coming across me in those instances might not think I'm nice...
For me, I treat people how I'd like to be treated, chat to anyone, I'm told I come across as relaxed and patient so I'm generally pretty well liked, I think, but I'm actually at a point in my life where I'm very comfortable being myself and if anyone doesn't like me I'm not bothered!