Also getting Raptor engines to work. The second Starship test was a massive achievement because the SuperHeavy Raptors all worked. That's an engine design with numerous advantages which wasn't practical when the Russians tried it, but might now be practical (certainly looks to be from the test) because control systems technology is small and powerful enough (i.e. similar to how modern fighter jets fly in aerodynamically unstable regimes requiring active computer control - we have them now because we have the computers which can do it).