Longevity doesn't make a player great though, else someone like Chanderpaul would be classified as a great.
Bradman didn't have longevity either (in terms of games played). By 2010/11 it was widely established that Dale Steyn was a great, the only question was where he'd finish amongst the greats. Case in point with Amla.
What makes Amla a great for me is not stats (he's got them anyway) but how he performed against the best teams of his era.
Against the greatest side Asia has ever produced he stood tall. Took his team to within 10 deliveries of winning a series there. He dead batted everything in the second innings the Indians had no answer to him. Even the great Australian team always struggled in India. Sangakkara struggled in conditions he knows against the greatest side we'll ever see for those conditions.
When the number 1 ranking was on the line against the best English side in a very long time, again he produced the goods. Did the same in Australia, made a tough pitch and a low scoring match look like candy with a swashbuckling knock of 196.
In Sri Lanka he enabled South Africa to save a Test and secure a series win. What made the series win impressive was how it was achieved. How many teams have played for a draw from day 2 of a Test match and got away with it?
SA would have been slaughtered in the media for "negative" tactics had they failed (perhaps rightly so). But they had a game plan and stuck to it. Amla was vital in ensuring SA avoided the follow in the first innings (130 from 300+ deliveries), and another 30 odd from 150 deliveries in the second innings.
He has played so many different innings at crucial times and in different conditions. That's what makes him great for me not stats.
Two of the best number three's of the last 20 years were Ponting and Dravid, I don't know if Amla is as good as them yet. But it can't be argued that Amla is more versatile of the two. Ponting was attacking and didn't care much about saving Test matches. Dravid on the other hand was on the cautious side and didn't grab the game by the scruff of the neck. With Amla you have the best if both worlds. I'd have him ahead of them both in my starting XI. It's not to say he may or may not be better, that's a discussion for another day. With Amla you can consolidate and attack at the same time. But that's just my opinion and shouldn't be taken as gospel.
Amla does have chinks in his armour just like any other player, no disputing that either.