That's Nintendo for you. They just keep releasing the same games constantly with new gimmicks and such each new iteration. Main games like Zelda and the main Mario games have it better because they come out less often and do innovate somehow, but stuff like Mario Kart and Mario Party is always the same old, same old. I personally don't care about Mario Kart until they take out the Blue Shell and make it FEEL fast. It's a racing game but it feels incredibly slow for me.
The more it progresses, the more difficult it is to make changes in a video game. It's hard to explain, but you need creativity. You know, these days, developers are afraid to change things because they can not know how the public will welcome it. I loved Mario Kart Wii. Here, we offer you something in HD, with new karts and new ways to play. I think Microsoft has fulfilled his part of the contract. Now, players must try to understand the new and fun in this game
I think it's been different and evolved over the years. It's a real fun game to play and even with small/few changes it still is great.
I have to agree with walker6 about not messing with the core of the game. With every MK incarnation it has proven its mass appeal time and again. As for Mario Kart 8, the news is that anti-gravity tracks will be incorporated which will allow you to drive upside down or vertically. Your kart's headlights will also turn on if you are in a dark area.
