Arcadey feel combined with the long breaths typical of Marathon campaigns, combat polished to an absolute fault and with its own identity, plain old good mapping and a satisfying arsenal. Everything garnished with a fairly good story that explains various new gameplay elements in an immersive way and provides awesome setpieces to play through, as well as great balance even and especially on Total Carnage. I see very little reason not to treat yourself to this scenario if you are able to.
A 35-level single player major conversion.
This scenario is difficult and action-packed; it features a full arsenal of new, powerful weapons, and more threatening enemies than you could ever want.
1.4 has new Lua features like a performance overlay, weapon buffs, a lot of little bug fixes; it also applies ammo capacity limits on Total Carnage. Requires Aleph One 1.5 or newer, probably!