The consistency, too.
When mine first installed, it was 3ms or so, it's up to 8ms now the network is more loaded, but it's absolutely rock solid.
I think there is such a thing as "threshold" bandwidth, for us that was about 15mbits up and 100 down, and it means that no matter what silly stuff people are doing (other than saturating the entire pipe), things just keep on trundling.
When I first signed up with Zen, they gave a lifetime price guarantee. They don't offer it any more (and the prices have actually fallen since!), but it was definitely a big breath of fresh air after virgin "press 1 to only increase the bill by £2.50 instead of £5" media.