From what I've read about Tron Legacy, the explanation for the different look is that the ENCOM mainframe has been evolving independently since the time of the last film. Hence, the drastically updated landscape, lightcycles, recognizers, etc. I don't know how close the final film is going to look to the concept trailer shown last year at SDCC (and, now, officially released by Disney, URL below), but if it's anything similar to it, it's going to kick ass.

The URL for the officially released trailers: http://www.flynnlives.com/media/video/0xendgame.aspx (two are downloadable WMV files, the rest are regular and HD QuickTime). They definitely blow away the bootleg FLV that's floating around the net, but by the same token, they don't have the excitement factor that it had (mainly, the reactions of the crowd to the knowledge that they're seeing the Tronverse and to the appearance of Jeff Bridges).