Greatest Software Ever Written 4227
Ask an experienced TeX user :) I confess I'm not familiar with Script. But if it had done everything Knuth needed, he wouldn't have bothered with TeX :-)
First, there is the richness and versatility of the TeX language (in particular, WYSIWYG systems like InDesign are drained of this power). Knuth has designed a great many domain-specific languages in his career. Many markup languages and macro languages have been invented for typesetting and text formatting over the years but I have seen none with TeX's elegance and practicality. This applies to both mundane typography and the specialised area of mathematical setting. Knuth knew the computer could transform the latter from a traditionally painstaking task for conventional typesetters into something every scholar could learn to encode in his own documents.
It has internal beauties. Many new algorithms were proven inside TeX - Knuth's optimal paragraph breaking scheme remains ne plus ultra; TeX's scheme for hyphenating English, another indispensable part, made other systems look cretinous.
Greatest Software Ever Written 4228
remove the half just bring us back. Would you rather have Metafont-TeX or vols. 4 and 5....? You only have 1 DEK, so it's...
Furthermore, the entire system is documented humorously and beautifully in The TeXbook and its own source code (a pioneering exercise in Literate Programming).
It helped a great deal that Knuth thoroughly versed himself in typographic traditions and was keen to apply his beloved computer science to a pressing personal quest, publication of his own books. He honed TeX over years of using it himself, and made the fruits of all this freely available. The formula had prodigious results.