Ubiquity of the word. For example, common words are more likely to be put up there, so a better AI would pick common words first. A worse AI will pick at random.
Figuring out what letters can or cannot be next to another. For example, "xz" doesn't happen ever in English. Same with eeii and other combinations. A smart algorithm uses that information to prune the search tree. A dumb one doesn't.
My criteria for good software:
Does it work?
Can someone else come in, make a change, and be reasonably certain no bugs were introduced?