I wanna pop in because I want to try playing with "fields":
http://wiki.guildwars2.com/wiki/ComboSo, for example, an engineer can drop a Fire Bomb, creating a Fire Field. He can then drop a Big Ol' Bomb in it to create a blast effect within the field that results in all friendlies in the area getting Might.
Or a Ranger can set a Frost Trap, which creates an Ice Field, and then a guardian can do a Whirling Wrath while standing in it add a Chill effect to all of his bolts.
These combos aren't really given much of a highlight that I could see in the game, so I never think to mess with them much. I knew I could drop a fire wall and then shoot through it to add burning attacks to my bullets but there's a lot of fields and things you can do with them and some aren't very obvious. I want to take a closer look at the fields I can create and see what effects I can maybe get some real use out of.