One thing TA and all of its successors kept was the ability to have units assist in construction. For instance, you have your commander plunk down a factory, then order the commander to assist with anything the factory is doing. Early on, this will be creating engineers (builders) and your first defensive units. Then you have your commander and engineers build/upgrade your level 2 factory.. rinse and repeat. It turns into a virtuous cycle.
The upshot of this is that it creates opportunities for prioritization (and opportunities for blowing past your income if you're not careful).
Oh, yeah, THIS was a huge one. As well as just how construction/payment worked in general. This thing costs you X metal, and this factory can deliver metal at a rate of x/sec... but if you throw in all these helper units, you can up that. Very interesting and unique twist.
You also didn't have to pay up front when queuing things, like you say with the opportunity to blow past your income, so your cash flow was the counter-balancing production speed limiter, but not in a way where you had to be constantly watching that bank account dollar amount before clicking "build."
The upshot of this is that it creates opportunities for prioritization (and opportunities for blowing past your income if you're not careful).