Trash talking AI

That depends on how you plan to do it.

If you're talking about the computer actually parsing what you type and coming back with witty responses generated in real-time, that is a pretty complex AI problem. If you can actually implement it, I think it's a great idea - try checking out facade (www.interactivestory.net) for an ambitious but IMHO still far from complete example of allowing you to type what you want, and having the AI characters respond in a relatively appropriate manner.

If you're talking about pre-set things for the computer to say (like "ha ha!" "you stupid bastard!" etc.) - this has been done before (Tribes, Unreal Tournament both come to mind), and I don't think it adds in any significant way to the game - after the first couple of games they just become annoying (of course, in multiplayer it's a completely different story, but you're talking about AI).

Not to be a downer, but imo your effort would be better spent in other areas. A trash talking AI loses its novelty after a few minutes, and that's when I'd start looking for the AI 'mute' button.

If you use this sparingly and know someone who does good voice acting, I think this is a fantastic idea. The best trash talking I've received in a game has to be system shock 2. Strangely, it is a megalomanic AI called shodan, saying things like 'you will do as I tell you, insect'. And the famous sentence:

Shodan: "Look at you, hacker: a pathetic creature of meat and bone, panting and sweating as you run through my corridors. How can you challenge a perfect, immortal machine?"

I like the text, but the real strength is in the way it actually sounds so be sure to look for a sound snippet somewhere. I think the way it is said is much more important than what is being said. I could hear shodan a million times and not get bored. (indeed I have). Command & conquer generals has a challenge mode that gives you some trash talk which I think is rather corny, but I still like the almost cartoonish voice acting and it did inspire me to try to beat them, even though initially I didn't like the game mode at all.

I think there's something to be said about games that trashtalk or intentionally try to antagonize the player. Whether it be the 'Run coward!' of Sinistar or even the mocking laugh of the dog in Duck Hunt, I think it adds another layer of immersion to the gaming experience.

When it becomes personal, you not only want to beat the AI but you want to friggin clobber it.

I rather liked the combat comments in Monster Seed and Final Fantasy 10. They weren't just trash talk though, there were also complaints, expressions of fear, battlecries, and funny error messages. I think the key is to make sure the player doesn't hear any particular comment more than once an hour, or even less if your game is long. They shouldn't hear any particular comment more than 6 or at most 10 times during the whole game.