Using something other than the recognised root nameservers for your DNS.
Alternative roots are of little value until a significant number of people have settled on one, which means the advocates of alternative roots frequently resort to mass-emailing sysadmins or installing scumware to try and get any significant number of people using them.
So far the most sucessful alternative root is new.net, which they claim works for almost 20% of the internet. That means it doesn't work for about the other 80% of the internet, which is pretty sad.
Another concern; most of the alternative roots are creating the same TLD's for themselves; .shop, .kids, etc.. in the unlikely event that someone starts advertising their website at 'http://www.pie.shop' and you even recognise it as an alternative-root domain (and most web users won't), which alternative root are you going to try?