The Blizzard developers had a lot to say through the panel and gave some great advice to map developers.  Blizzard wanted to make clear that they're not out to complete with custom maps with these releases.  They did say that having been through and designed some more custom maps they've realized some things that could use some improvement such as the unit inventory system.

• Blizzard would like to reveal the art tools they use at some point to aid developers in developing their maps
• An easy customization to focus on is tint color, you can very quickly customize a classic unit.
• Blizzard will slowly convert all Warcraft 3 and most World of Warcraft units to be included with Starcraft 2.

Blizzard map developers suggest putting a major focus on:

• Game type
• Fun factor - Too many mechanics can ruin a game, focus on not deluting the fun
• Easy to learn - A good example is a tower defense map, you start off with 1 unit and very few options
• Feedback - Go online and pursue a community (our Starcraft 2 Map Database can be a valueable tool)
• New Player Experience - Players are quick to blame a map for bad gameplay.  Intro cinematics are great, but experienced players need a way to skip them

Blizzard DOTA

Left 2 Die

Starjeweled

Aiur Chef