Some initial reactions, in no particular order:
< $500 limits your choices to Walmart/department-store bikes (not recommended), used higher-quality brands, entry-level BikesDirect bikes, and possibly some "bike shop" bikes (entry-level candidates from Fuji, Raleigh, etc.). Disc brakes are pretty uncommon on road bikes. The Windsor Tourist comes with a rack. The Mercier Galaxy is oriented toward touring and accepts racks and comes with slightly wider tires than most road bikes; this would be the top candidate if I were in your shoes and wanted to meet (much of) your criteria. I would stay away from the 'Schwinn' Varsity and other Walmart Schwinn bikes (these are branded Schwinn but made by a company called 'Pacific Cycle'). The Cafe Latte does not have drop bars, so I would rule that out since you've said you want exposure to the 'sport of cycling'. Schrader valves are less common on road tire tubes, but can be found on some, more often on tubes intended for wider city/touring tires. They aren't adequate for the higher pressures used in most road tires, so I wouldn't get hung up on this. If all you have is a Schrader pump, valve adapters are available for a buck or so anyway. Also, you may not be able to keep the weight under 25 pounds, particularly if you're looking at wider tires and racks and a variety of uses, and as a beginning cyclist and commuter this shouldn't be a big deal anyway.
On a personal note, I purchased an entry-level road bicycle from BikesDirect and have been very happy with it (having been using it for a little over a year now as a regular commuter and recreational cyclist).
Important things are that you get a bike that fits you and that you become familiar with basic bicycle maintenance. Good resources for bicycle maintenance and repair instruction include the following most popular books, as well as
www.sheldonbrown.com:
[1] Zinn & the Art of Road Bike Maintenance
http://www.amazon.com/Zinn-Art-Road-...1038743&sr=1-2
[2] Park Tools Big Blue Book of Bicycle Repair, 2nd Ed.
http://www.amazon.com/Park-Tool-BBB-...1038702&sr=8-1
[3] The Bicycling Guide to Complete Bicycle Maintenance and Repair, 5th Ed.
http://www.amazon.com/Bicycling-Comp...1038743&sr=1-1
Additionally, this question has been brought up many times before on the forum, so feel free to search the forum for those (not for the particular bikes you mentioned, but for first-time buying advice and recommendations generally). Many of them cover important points I have not covered. I think when dealing with entry-level bikes, the important thing is that you get something of quality and that fits you properly. The components on all entry-level road bikes are very similar in style and quality.