I have much more important things to work on, but I just need to take a second to reply quickly to a lot of things that I'm seeing on the web about the lack of stuff to do in Downtown Atlanta after hours. I could go on and on about some of the stuff that I am reading but I just don't have the time right now so I'm just gonna get to the point. These perceptions are untrue. There is a very diverse set of things to do within a 2 mile radius downtown.

To the person coming out of Phillips Arena or the Georgia Dome wanting something to do, all of these places are within 3 blocks of Phillips Arena:

Luckie Food Lounge - Very large, very trendy joint.
Stats - The ultimate sports bar. Has a freaking gigantic ticker over the main bar and a great rooftop deck.
Wine Loft - Haven't been, but it looks nice. Also looked to have a fairly serious dress code.
Rise Sushi Lounge - Great spot. Even if you're not into Sushi you'll find something to like. They also have a menu that they do for half off on Thursday Friday and Saturdays after 10.
Taco Mac - Phillips Arena - It's Taco Mac, that means tons of flat screen TVs and 80 beers on tap. Hundreds to choose from.
McCormick and Schmicks - CNN Center - Has a nice bar area with booths and stand up tables.
Latitudes (In the Omni Hotel) - Big bar with a large sitting area. Great views of Centennial Park
Thrive - Uber trendy and sleek, I hear that the sushi is incredible.
Maxim Prime - It's a steakhouse but it has three bars, one on the rooftop with what I think is one of the two best views of the city from any vantage point.
Sabor - Loungy Bar.

If you want to broaden your radius, you can go a few blocks over into the Fairlie Poplar District and hang out with the locals at:

Sidebar - This is the "Cheers" of the Fairlie Poplar District
Slice - Stylish joint where you can get some decent pizza, sandwiches, and pasta. Good spot to people watch, especially during Fetish Night at The Mark.
The Mark Ultralounge - The only dance club in Fairlie Poplar. Extremely diverse events from night to night.

If you want to dance, the Mark is having a "Unicorn Helmet Night," and you're not scared of black people, there is much space for dancing at Kenny's Alley in the Atlanta Underground .

A few blocks south of Phillips is Castleberry Hill. A cool neighborhood with lots of lofts, art galleries and night spots:

Slice - The original. If you're into crowds, go here.
M Bar - Simple, chill spot. It actually has 2 bars so don't be fooled when you look inside.
255 Tapas - Nice sized joint with a fair amount of TV sets. One of the few Tapas hustles where I don't feel totally ripped off.

Want some live indie music? Go over to Lenny's in the Old Fourth Ward (ish)

Also, in he Old Fourth Ward are a host of other great places on Edgewood Ave:

Cafe Circa - Interesting menu, chill decor, live music twice a week, no hats or t-shirts. Wear a collar.
The Bureau - A place where you can kick it with the boys and impress a lady on a date. Awesome staff. Tell them Rashid sent you. They'll probably laugh and go get me.
Noni's Bar & Deli - Italian Comfort Food. Really chill environment. Great date spot.
The Harlem Bar - Great menu, live DJ on the weekends, kitchen stays open til 2:15 AM!!!
The Edgewood Corner Tavern - It's a bar with bar food and pool. Get to it!
Over Da Edge - I have been to this spot a few times and it was okay, just a little far off the path. The wings were pretty good. They describe their clientele as "sexy" so do what you must with that.

All of these places are either walking distance or a short cab ride from Phillips Arena or Five Points. I've actually left off the Sona district (which apparently is now considered part of downtown), the Hotel District, and much of the Marietta St. Corridor so there is plenty more stuff to get into. Downtown isn't perfect, but to say that there is nothing to do is ignorant at best. Check out the CAP site for more nightlife destinations (over 50 in downtown alone though I think the Masquerade is a bit of a stretch). Their list needs to be updated, but that's mainly because they are missing a few spots that have opened recently.