A few things I never really understood...

[CyberCubed]

Originally Posted by Tarris Vaal

* What exactly was the difference between the normal Kraang droids and the ones with the blue 'shell/shield'?

Nothing, it was just a different design to make the fights look more varied. The Utrom used silver droids we see in Season 5.

Where did all the human foot ninja in NYC go?

You mean the ones in Season 1? They were likely all disbanded or sent back to Japan to re-train. Remember Chris Bradford was the one who trained Foot recruits in the dojo in this show, but after he was mutated his dojo was abandoned.

We see in late Season 4 Karai brings in fresh new human recruits from Japan.

Obviously for budget reasons the streets are pretty deserted most of the time, but why did we so rarely get any busy street scenes?

The streets are plenty busy from Seasons 2-5. We see cars driving in the background and plenty of pedestrians. During Season 1 it was likely budget constraints.

The Kraang upgraded their weapon platforms regularly (flyers, spider walkers, the bio droids), but why did they never upgrade their own android bodies? The Foot managed to do so, the Kraang seemed to never bother.

Because the Kraang are morons. Kraang Subprime even mocks them in the fact that they can't speak normal english when they've been on Earth for thousands of years.

What was the deal with the Dregg Robot? Or Dregg dying after being 'spaced' only to return later?

This was never really explained, I assume the Dregg in "The Evil of Dregg" episode was a robot, whereas the real Dregg was off attacking the salamandrian homeworld. We see Dregg again in the Tokka episode attack the Turtles, and after that he's ejected into space and frozen by the Triceratons, however he didn't die.

Then in Season 5 Dregg invades Earth for one last attempt to destroy the Turtles and conquer their world, and then he dies for good when his ship blows up after Newtralizer energizes the core and they all explode.

If the Kraang were stealing scientists (plural), then why did the turtles only ever see and successfully rescue April's dad? Where were the rest of them?

Other scientists were probably either mutated or experimented on like April's mother and disposed of.

Where does Splinter get the turtles armoury from?

You can ask the same thing here as any TMNT incarnation. Where do the Turtles get money from to buy food either and order pizza? It's never explained in any universe.