Look, you're going to have a few differences on any game within a given subject made by different development teams, this is a given. If another company made an espionage game made about an agent entering enemy territory with nothing but a stun pistol, basic med kit and a can opener, it would be obvious that they were copying Metal Gear Solid, and having different weapons here and there wouldn't mitigate anything. I made a point to list the essential gameplay and structure of GT and Forza for a reason. Both of those games aren't like anything but each other.
Microsoft is doing this because they're attacking SONY at their strength, which is what monopolies do when they want to conquer a market, or at least gobble up a good piece of it. Gran Turismo is SONY's premiere best selling game series, practically the mascot for the Playstation brand. M$ throws money around to acquire popular exclusives like Final Fantasy to weaken SONY's position in the market, and since they can't buy Polyphony, thank God, they copy them. If MS or EA ever wrestled Polyphony away from SONY, assuming they could, they would muck up Gran Turismo the way EA muddled Shift when it could have been a good simmish racer on consoles. Monopolies are bad, they stifle competition, innovation and overall quality. But I digress.
But having said that, Forza 4 looks good. Most remarks in the Forza section are very positive. I'm steering away from (pun intended) the latest vids so I can experience the game with unhyped eyes and sort through the goodies with few spoilers. This might be the first Forza in which I enjoy hotlapping as much as I do racing, which is something I experienced in Gran Turismo since getting GT4.
Still, I have to say that the "game" I'm the most excited about is GT5 2.0.