Creatures with no abilities get +2/+2.
5/1/2007 Muraganda Petroglyphs gives a bonus only to creatures that have no rules text at all. This includes true vanilla creatures (such as Grizzly Bears), face-down creatures, many tokens, and creatures that have lost their abilities (due to Ovinize, for example). Any ability of any kind, whether or not the ability functions in the on the battlefield zone, including things like “Cycling ” means the creature doesn’t get the bonus.
5/1/2007 Animated basic lands have mana abilities, so they won’t get the bonus.
5/1/2007 Some Auras and Equipment grant abilities to creatures, meaning the affected creature would no longer get the +2/+2 bonus. For example, Flight grants flying to the enchanted creature. Other Auras and Equipment do not, meaning the affected creature would continue to get the +2/+2 bonus. For example, Dehydration states something now true about the enchanted creature, but doesn’t give it any abilities. Auras and Equipment that grant abilities will use the words “gains” or “has,” and they’ll list a keyword ability or an ability in quotation marks.
7/1/2013 Cipher grants an ability to creatures, meaning the affected creatures would no longer get the +2/+2 bonus.