At the risk of restating something already said I will add that I too have noticed that BTS demands a different play style. I play on emperor, and back in warlords I could win by warmongering early and often, playing peace just well enough to stave off total bankruptcy. I would fight 2-3 wars before rifling, then sit back with my larger territory and cottage everything to beat the AI's into space. From what I used to hear around here about the importance of warmongering on emperor+, I think that that is an experience that others had as well. In BTS I quickly realized that warring early and often is less effective. The AI builds more units, detering those early rushes, and more importantly the AI builds a variety of units and seems smarter in its unit counters. This is not to say rushing is impossible, but the combination of a smarter, diverse, more militaristic AI, along with the dip in quality of starting locations in BTS (there is never any food to whip up that axe rush), combine to make rushing a less viable strategy. With the AI tech rate slowed by peace and unit maintenance, playing peace has a higher chance of success at a higher level, which I think is great!
Of course warring still works, but it is best done mid game once you have achieved some kind of tech lead. Plus, I fought my first real defensive war in BTS, when immortal Aggressive monty declared war on me as boudica. I got to build my first Dun, and it was fun to actually feel threatened as monty's 4 25 unit stacks marched into my territory. I didn't have much of a tech lead, so it really felt like I was fighting for my survival.
I'm in no way saying I think the BTS AI is perfect, and espionage definitely needs some work, but for me at least, it has made me broaden my play style a bit, which I think enhances the game.