It all depends on the table situation: If the watering hole is overloaded then intelligence isn't useful. If the watering hole is sparse and you have a -3 to throw in there intelligence is great, especially if you have cooperation. (Remember: You don't win Evolution by eating a lot of food, you win Evolution by eating more food than your opponents. If you eat 2 using intelligence and everyone else can only eat 1, that's better than you eating 6 and everyone else eating 7)
An intelligent carnivore is near impossible to defend against unless you spent a lot of cards just for defense, so building one can force your opponents to focus on things other than eating. On the last round an Intelligent carnivore can cost an opponent a lot of points if they can extinct a species.
Early in the game Fertile tends to be really good, late game it tends to be useless.
The designer has stated that Cooperation is a little more powerful than the other cards, but other than that they're pretty balanced and depend on the situation.
The stats for the game show the average number of each trait played by winners. The highest are:
Cooperation: 1.34
Carnivore: 1.14
Long neck: 1.14
Fertile: 0.98
Foraging: 0.93
So I guess a Cooperating Long Neck Carnivore must be the most OP species. :p