What is the significance of the 10,600 lbs MTOW limit?
It's nothing more than playing catch up.

If company X managed to build an aircraft for a certain mission at the lowest ever MTOW. Company Y will try to get down to that MTOW to be able to compete. Lowest is good if the mission is the same.

Check the MTOW of the Airbus A320-100 and the Boeing 737-400, to save you time, they are 200 pounds apart. But the A320-100 didn't sell well, the A320-200 did. Guess what, the 737-800 matched that weight.



Here's why:

There's something called Weight Factor.
$$
\text{Weight Factor}=\sqrt{\frac{\text{(MTOW in kg)}}{5}}
$$

The Weight Factor determines the route charges.


  This charge takes into account the distance flown and, less than proportionately, the aircraft weight.


That's why it's vital for competitors to have similar MTOW's. So they can advertise similar operating costs. MTOW is also used in airport fees.

The only interesting thing about 10,600 lb / 4,800 kg is that the weight factor is kept a tad under 10.00 (two decimal places are used). Now, is there a higher charge for a plane with WF at or above 10? I scoured the internet, there doesn't seem to be WF brackets set that way, to my dismay.

Ryanair has part of their fleet tailor-made to an MTOW of 66,990 kg (WF ~36.60) for similar reasons apparently, or they're just being cheeky.

It's also why some airlines don't choose the Extended Range. The 777-200 airframe and its ER version are the same (no auxiliary fuel tanks).

The differences are extra certification for higher MTOW, and the same engines but certified for higher thrust. Airlines can request a conversion to ER if they open routes to farther destinations.