Looking for a special occasion restaurant that is within walking distance or short cab ride from the Marriott Marquis if possible. Not familiar at all with Atlanta. Will be there for a convention without a car and will be celebrating my husbands birthday. Loves steak, but not a must. Thanks!
Not walking distance, but you wouldn't be that far from:
Rathbun's/Krog Bar/Serpas (haven't been to Rathbun's any time lately and it may have slid, but it fits the special-occasion requirement)
One Eared Stag
...and even Miller Union if you really stretched it. The steak options may vary.
In downtown, Nikolai's Roof or White Oak Kitchen (with the disclaimer that I had an awful customer service experience at WOK in May, but I would be willing to try it again). There's a Morton's close by as well, since you mention steak, although you can probably get that where you live.
In Midtown, Ecco and Empire State South are good.