Here are my thoughts on some of the proposed cities:
Atlanta
Present: I really liked CMP here. The weather was almost always beautiful, the subway provided good public transit from airport / around downtown area, and there was an abundance of parks, attractions, food courts, restaurants, hotels, and more downtown. The fact that ATL is a major airline hub makes is really easy to fly into. I would prefer a 1 hour longer flight if I can fly direct. The walk from the pits to the stands was a bit long, but the convention center was absolutely HUGE and would provide a lot of room for future growth.
Future: Why is ATL looking to replace what appears to be a perfectly fine stadium with a new one? It would be one thing if the stadium was 70 years old and requiring excessive maintenance, but it seemed relatively new.
A future benefit to Atlanta is that they are currently building a streetcar line downtown connecting all the major attractions together. Hopefully this will provide more convenient access around the downtown core.
St Louis
Present: The weather here is meh in April - the past few years have frequently been stormy, rainy, and cold ish. The Dome / convention center is nice, but there aren't that many options for food in downtown STL (no large food courts / malls in walking distance of Dome). Public transit is decent around downtown and to airport.
The downtown area seems "cold" and "dead" - like half the buildings are completely empty and there aren't many people walking about on the sidewalks. More activity and things in downtown would increase street life and make the area seem safer (more people around = people feel safer).
Future: If the Championship event continues to grow, finding enough space for pits will be an issue.
Indianapolis
Present: I haven't been to downtown Indy, but from the descriptions it seems good and are looking better for future economic growth. The stadium / convention center are close, but may require ramps again. While it may be nice for those in driving distance, flying into Indy is not particularly easy and usually requires layovers.
The lack of real public transit is an real hindrance (a temporary shuttle bus is a band aid). Having light rail / subway lines between the airport, downtown, and suburbs greatly improves how people and teams can get around and offers more flexibility in hotel accommodations if hotels in downtown are too expensive / already booked. Every time I go to IRI I spend a fairly large amount of money on taxis; I would much rather spend $2.50 on a train ticket
for this.
The weather seems like it would be comparable to St Louis.
Dallas
This was brought up in this thread as an option, but there is no real arena next to the fairly large downtown convention center. There is a mini "arena" in the convention center, but that barely held the double Dallas regionals in 2012. The public transit offering is decent, but doesn't yet connect directly to either Dallas airport (currently you need a shuttle bus from the Orange line to both Dallas Love and DFW airports). The Orange Line is planned to directly connect to DFW in a few years, and they are expanding the downtown Dallas streetcar lines. The DFW airport is a major hub and easy to fly into.
A lot of the attractions (sports arenas, the original Six Flags) are all in Arlington (15 miles west of Dallas) and difficult to get to without a car. The weather here in April is usually nice, but with an elevated chance of strong thunderstorms.
Despite living 45 minutes away from downtown Dallas (gotta love 75 mph speed limits), I think Atlanta is the overall best choice for the CMP location.