I often interview and it depends on what the role is as to what I expect of the interviewee.
Call centres/office works, definitely clean smart office wear, you don't have to come suited and booted, just clean, smart and obviously made an effort with your appearance, it tells me you are interested in making a good impression.
For some more hands on roles, like labouring, production line etc, smart casual as a minimum, no jeans, cheeky logo t-shirts or trainers. Clean smart etc.
Having said that, my brother in law once went out for a high flying job in a short sleeved shirt and chinos that looked like they had been slept in. I asked him if he wanted me to iron his clothes and he said no, they want him for his brain and not his appearance. He did get the job.
I think if you are borderline, your appearance can swing it either way for you.
My husband is an engineer and he goes suited and booted to job interviews and more often than not the interview panel are in jeans and t-shirts because its a practical role.
Someone I know in IT, once had an interviewee turn up as a Trekkie, in a full lycra Star Trek outfit and he never batted an eyelid about it. But the guy didn't get the job and not just because he didn't have the skillset, but because it was a client facing role and they had concerns ab out what he would wear to see clients.