I know it was said earlier, but recognizing the threat of a leader's current or future UU is almost as important as recognizing more direct threats.
For example, and it's not evident in the current ALC, but if you're playing as Egypt, and you end up near Mansa Musa, Alexander, Cyrus or either Khan (arguably Monetzuma as well), then the probability for success with a War Chariot rush is greatly diminished.
Likewise, if you find yourself near Tokugawa, you want to either fight him before he can produce Samurai (Macemen), or after Samurai are obsolete (prior to Samurai is generally safer).
Civs with late game UUs, such as America or Germany, are much less a threat than civs with early and mid game UUs in terms of such rogue attacks (though they generally make up for it with average or above average traits).
Similarly, if you find that Mansa Musa, Alexander, Cyrus or either Khan is on the opposite side of the landmass from you, you're probably safe from them making potent use of their UU.
As well, some midgame UUs are less than spectacular. Musketeers, for example, are the same strength as Musketmen (the unit they replace). Both units are, essentially, Midieval units, despite arriving at the beginning of the Renaissance era. They're generally considered weak because they are quickly made obsolete.