Google now looks at the length of time a domain is registered for. Legitimate(by rules) domains are more likely to be paid for several years in advance. While shady(doubtfull) domains are rearely registered for more than a year in advance, since the owner knows they are likely to be penalized anyway. I recommend you sign up or renew a domain for at least 2 years, preferably more. Legitimate businesses are in it for the long haul( pull) anyway.
Google may also take into account the rate at which whois information and DNS Nameserver is changed for a given domain. Domains that frequently change their hosting servers and registered owners may be flagged as possible spam domains.
Lastly. Google appears to give more weight to older domains ( and hence older sites when ranking sites, all other things being equal.