What's new in Vegas - and what's open Christmas?
You don't indicate a price point, but a few ideas:
Raku is (almost) universally loved here. Considering how popular it is, it is very reasonably priced and open late: http://www.raku-grill.com/
Namaste is right next door to Lotus of Siam and has received much love here. I'll finally have a chance to go this month -- among people I know, some Indian, it's considered to be the best Indian ever in Las Vegas.
On the high end, Bar Charlie, within Restaurant Charlie at the Palazzo, is very pricey but might have the most universally positively reviewed food of any of the expensive restaurants in Las Vegas here. The food is Japanese-based, and includes raw fish, but I wouldn't consider it a "sushi restaurant." Here's a link to the menu: http://www.palazzolasvegas.com/assets...
The Aria hotel, located between Bellagio and the Monte Carlo hotels will be opening next week. I think they have 13 restaurants and a bunch of those have big names attached to them. I wish they had their menus (prices $$) up because I'll be staying at Aria next weekend, but am to afraid to make reservations without knowing the costs! http://www.arialasvegas.com/dining/fi...