Not sure of the significance: https://en.bitcoin.it/wiki/Blocks#How_many_blocks_are_there....

    What is the maximum number of blocks?
    
    There is no maximum number, blocks just keep getting
    added to the end of the chain at an average rate of
    one every 10 minutes.

    Even when all 21 million coins have been generated?

    Yes. The blocks are for proving that transactions
    existed at a particular time. Transactions will
    still occur once all the coins have been generated,
    so blocks will still be created as long as people
    are trading Bitcoins.

Well, I guess it's a continuation of Bitcoin mining being ahead of schedule. Discussion https://hackernews.hn/item?id=5780861 and since that article is dead, here's another one http://www.dailydot.com/business/bitcoin-mining-55-years-ahe...

It's just a nice round number. The next halving of the coinbase reward is still far off.

Is there something notable about this block like a change in mining reward?

With a relatively controlled block rate of 6/hour, can't we predict when almost any block will be mined?

The long term averages are showing slightly more than 6 per hour. Over the short term the spacing between blocks varies pretty wildly. I sure would not want to rely on a clock powered by Bitcoin block times :-)

I'm not into Bitcoin. So could someone explain the significance of this to us laymen?