I think the fact that they are pushing Lightning parallels to the other female characters in current video games: Tomb Raider and Remember Me. It is nice to see a shift from male main characters to strong female leads.
Between Terra, Celes and Ashe. you can't really call it a shift when it already happened. Let us not forget other memorable female characters like Agrias and Beatrix.
The major difference is that none of the female characters I've mentioned were shoved down people's throats the way Lightning has been.
I don't think Lightning is replacing Cloud from FFVII (Cloud is one of the most iconic character in FF series)
From what I have heard, the order from the higher ups was to make Lightning heavily based on Cloud. Wada also liked the character.
I'm not to keen on what has been revealed. It'd be one thing if they had thrown in an homage/parody character with a ressemblance to Lightning. Inserting her into the game for cross-promotion purposes just doesn't seem right to me. Items and weapons from other FFs are nice and all, not to mention references like XIV's own version of the Crystal Tower, Gold Saucer and using names like The Enterprise and Tiny Bronco. Porting character from other games into this one just leaves a bad taste in my mouth.