Ah, I see what you mean there. Before I had sat down to play FTDM, I thought I was pretty good at min/maxing powers and aiming for the best case scenario. So I've learned TONS in the 9 matches I've played in, especially the action economy in 4e. I know it makes some feel like 4e is only a tactical game. Our goal with FTDM was really to take that tactical combat to the extreme. To feel the rush after gibbing an enemy, being able to deal 30+ damage in a round (even with a low chance to hit), and revel in pushing the rules as far as they could go. The maps and rules are being playtested and modified continually, so multiple character builds can be built to take advantage of the map features. The next map is a great example; you face the choice of a lava filled room or being attacked by the Cyberdemon hazard at the end of the round. Therefore, tiefling and genasi have advantages with fire resistance, but they still can be killed off in a single round with a well placed backstab by the human rogue who used ambush trick to waltz up to them and put a knife in their ribs!
And to give you some more fore-knowledge for optimizing, the structure of the respawn power is key to FTDM, as all encounter powers refresh just as if you'd taken a short rest. So many players build around their encounter power, and go from there. Luckily, 4e gives you so many choices, there are a heap of builds which would never work in a normal campaign but are godly for a round or two (i.e. bladesinger, certain genasi, hybrid cleric barbarians) between respawns.
In terms of a theme, most groups just use it for flavor, to add to the excitement. Normally, once the teams are formed, everyone works out a team name for the match, choses an image token for the map, and it builds from there. We recently had a Marvel vs JLA heroes matchup, with players rarely breaking character, trash talking (especially the player who ran deadpool!), and adding humor to the competitive atmosphere.