Ok, your suggestions sound nice. assume everyone does either of these things mentioned above and I`m one of the few that has never tried that out, right? Also, higher difficulty levels don`t make my game much more challenging. Yes, I`m behind in research and soldiers and everything else on higher levels, but it`s still all about pressing the end turn button and managing tiles. Is BTS a must?
I'm not sure if you were responding to my post or to someone else's post...
Not everyone wants an aggressive AI, so not everyone is changing settings to get a more aggressive AI. Some players find it annoying when they are getting attacked and just want to build an empire. So they'll complain about attacks on the forum. Other players don't want a builder game and want to be challenged militarily by the AI and so they'll complain about a passive AI.
The aggressive AI setting is a bit of a no-brainer for players who want to be challenged militarily. Of course, it is useful for those who want a more aggressive AI opponent and not useful for those who want a peaceful game. However, the setting caused the AI to be more annoyed to the human player in vanilla civ4 and Warlords. It didn't necessarily make the AI more aggressive. Of course, a more annoyed AI is more likely to become aggressive, but it's not the same as an AI which invests more in military and will attack weaker civilizations. That's what happens when you activate the aggressive AI setting in Beyond the Sword.
In Beyond the Sword, the AI is also slightly better at waging war. It's still nowhere near the capabilities of a competent human player, but that's also impossible to program.
About the difficulty level: If you play vanilla civ4 without the aggressive AI setting, then a higher difficulty level might not make the game more challenging militarily as the AI might still not attack you very often. However, the AI is programmed to have a bigger chance to attack weaker civilizations (weaker in military strength, not weaker in score). At higher difficulty levels, the AI is more likely to have equally or more advanced military units and can produce units faster. So at a higher difficulty level, you're more likely to be attacked as you are relatively weaker and the attacks are also more dangerous.
In Beyond the Sword with the aggressive AI setting, a more advanced strong neighbour can be dangerous. It will attack you if you can't keep up in the military buildup (except when you can become friends with them, diplomatic relations: friendly). It will build up a strong army with artillery and attack units in a big stack and then attack you. If you don't expect the attack, then it will probably capture some cities before you can counter the attack. Depending on your skill and the difficulty level of the game, you might still turn the tide, but it's not necessarily easy.