IIRC it came from turning cities into discrete world zones, larger than their actual map size, and they couldn't figure out a neat way to let that work if players could come in from any angles other than a few clearly defined gateways.
But getting rid of player-controlled verticality makes things much simpler, which probably helped the decision.
That perilous yellow haze in the photo proves it to be the work of that dastardly paragon of scientific cunning - the Fiendish Dr Food Munchu.
There's a nice deconstruction of WWZ ongoing on SV.
Lets Read: World War Z: An Oral History of the Zombie War
It takes the form of a in-universe read through of the text, challenging the Hard Men Saved The World narrative as a piece of propaganda to support the US Junta and their overseas allies, by a survivor in Bristol. It reinterprets many events to actually make some kind of sense - North Korea isn't a spoopy megabunker, but close enough that propaganda can spin it that way. Yonkers was a deliberate destruction of units and officers who were considered unlikely to support abandoning half the country to die and couping the government.
It will get everything killed. The one time it works out for you will be totally sweet though.
AoE spells are just fun - Colour Spray, Grease, Web, etc - but will wreck your fighters unless you place them carefully (and the AI is pretty suicidal). Eventually, there are a lot of low-threat opponents, which means even the low-level ones stay useful the entire game.
Usually the most dangerous threat is anything that can throw bullshit magic back at you - so, monsters - and then worse, anything that can do that magic and has protections against your magic - mostly wizards. Most of those fancy monsters want you to stock up on a couple of status-healing items when you notice them for sale, just in case. Wizard fights become about comprehensively dispelling their protections and then them instantly exploding from Minsc slapping them with a greatsword.
Later, in BG2, things get pretty damn unfair (in both directions - a lot of near-impossible fights are totally negated because you scrounged up money in the starting city to buy an overpriced shield).