How to Create a Popular Battle.net Map:
Method #1
Anything to do with World War II will do. It does not matter the quality, as long as you have World War II in the name of the game and World War II themed unit names, people will play this map. This mysterious outcome is not limited to StarCraft however, it affects all gamers. Here are some example of other World War II games which are popular despite being of horrendously low quality: Medal of Honor, Call of Duty, Brother in Arms, Medal of Honor, Close Combat, Battlefield 1942, Medal of Honor, Combat Elite, Iron Aces, Commandos, WWII Fighters, WWII Combat, Medal of Honor.
Method #2
People love to see other people get hurt. The closest thing to this on StarCraft is to crash the player's computer. Present the player with a simple situation in which, should they fail, they are immediately crashed. People playing this for the first time should always fail, while simple knowledge should prevent them from ever failing again. This way, they will host the map again so they can see other people crash while knowing they won't be crashed themselves.
Method #3
Rehash an old concept. Cat and Mouse has been done a million times over, and every time someone makes a new map, it's even craptier than the last one. Now, after several years of this happening, it's going to be extremely difficult for you to make a map that is worse than the ones already out there, but I imagine it's still possible. Again, using Cat and Mouse as an example, you could make a map called "Cat and Mouse and Fleas and Ticks and Rats and Bats and Gnats and Ants and Hats and People named Pat."
Method #4
Create a new, original idea. This is the only respectable way to make a popular map. Notice that I have not said that the map must be good, or that it will be good. It's perfectly acceptable to take a new idea and make monkey feces out of it; it will still be popular. Be warned however that there are three outcomes to this scenario. The first, people will take your map and make even craptier ideas out of it. This is how bounds came into existence, as well as all the derivatives of Panzer_Kavalier maps. The second outcome, people will take your map and make a good, fun version out of it by using "Note #1" below. The third outcome is that someone will take your map, make a fun, quality map out of it, and only a select few people will play it. Those people will laugh at you and the "newbs" who play the crapty versions.
Note #1
One way to add a lot of popularity to your map is to give the player an extreme sense that what he or she is doing really matters, and that it will effect the outcome of the game. However, this only works if the exact opposite is true. People need to feel that they are playing a role in the outcome, but in actuality you need to do everything in your power to make sure that the outcome is either always the same, randomized, or dependent on the intelligent people leaving the game and thus tipping the scales to one team or the other. This effect is commonly seen in the "LotR"ish games in which the player is given a massive amount of units to throw at the enemies defenses, which have 50kajillion health and do one hit kills. There is no chance of the outcome ever being different, but the player seems to think so. Right on!
Note #2
People love to hear things. Adding sound effects to your map automatically makes it seem professional, and everyone will be utterly amazed by how totally awesome you are. It's especially effective if you make the sounds take place when something especially great happens, like an enemy getting his ass handed to him. The problem with this rule, is that it directly conflicts with Note #3 below.
Note #3
People don't like to download large maps. It doesn't matter how good your map is, if the file size is too big, people aren't going to download it. Granted, pretty much everyone has high speed these days, but Battle.net slows that high speed down to about "medium" speed. On the other hand, most people with high speed can't get on Battle.net anyway because they're too freaking stupid to use their router properly. If they do get on Battle.net, they clog the games list with High Latency games or complain that they can't join your games. So with this logic in hand, if you're actually capable of playing a game with someone, he's using dialup. Don't make him download five megabytes.