Jossed: The Epileptic Tree about Yes Man's "assertiveness" upgrade being meant to set him up as The Starscream. Word Of God says that Yes Man was not foreshadowing a future betrayal. Quite the opposite, it was him being Genre Savvy and working to prevent himself from being turned against you in the next round of musical chairs.
Refitted For Sequel: The DLC is based around this. Originally it was going to be another partner named Ulysses, having been part of Caesar's Legion. He was removed from the main game, but was expanded to being hinted in the first DLC and then becoming a major character in Lonesome Road.
Fallout Van Buren was canceled, but some minor plot elements and Caesar's Legion were incorporated into New Vegas. The DLC took even more inspiration from Van Buren, particularly Old World Blues and Lonesome Road.
There were early plans to extend the game beyond the final battle, but the amount of variables (ie, programming a Mojave with the victory of four possible factions and numerous minor details) made it too impossible to program within the deadline.
Ulysses, the antagonist of Lonesome Road, was originally intended to be a companion in the main game, but was cut due to time constraints. He would have been unique among the available companions as actually being sympathetic to the Legion and especially untrusting of the NCR. Much of that personality remains in his Lonesome Road appearance, but if he'd been in the main game as originally intended, there might've been motivation to actually work with the Legion. As it stands, you can earn a pair of Luck-boosting sunglasses...but you lose the loyalty of CraigBoone and ArcadeGannon, and you'll likely end up killing at least a few factions/settlements that you actually liked. Of course, Ulysses could be persuaded to reluctantly serve the bear or another cause and will tells you about the battle of Hoover Dam.
Similarly, Word Of God also states that there were originally more Legion territories planned east of the Colorado River, but they were cut due to time constraints.
Originally, if you destroy the Securitrons under the Fort, you were originally going to be forced to fight a seriously pissed off Victor the next time you enter the Lucky 38.
Many instances of cut content (including the example directly above), have since been released in a mod series appropriately titled "New Vegas Uncut".
"Caesar can quote Cato to his purpose" is a paraphrase of a line from The Merchant of Venice: "The Devil can cite Scripture to his purpose." (Though Shakespeare is pithily reflecting much older wisdom.)
"Am I playing Vergil to your Dante?": The Divine Comedy, where Vergil serves as Dante's guide through Hell.
"Victrix causa diis placuit sed victa Catoni." is from Pharsalia by Lucan. Translated it is simply: "the winning cause pleased the gods, but the losing cause pleased Cato." Considering its context, it fits very well.