You've clearly not played very many good FPS's. Balance is the most important thing when it comes to multiplayer FPS maps.
The maps look good, sure, but balancing especially on the first part of most of the maps in rush mode is terrible. If you have a few slow loaders, or just get held the first few times, the defending teams can absolutely camp the living hell out of attacking teams very easily.
I think the guy may just mean something different when he says "BC2 has good level design." Maybe he is thinking of how the maps are large, pretty to look at, and have destruction and a lot of detail (given the size of the maps).
However, I agree with you in that when I hear the term level design, I think of balance. Rush mode certainly has some balance issues. A balance issue is when the map has an influence on which team will win in a round.
I play a lot of squad rush because I compete in tournaments, and that gametype makes the outcome of a round completely dependent on which side you are on. Good matchups between teams rarely see a second bomb destroyed on Port Valdez SR, Atacama Desert SR, or Valparaiso SR. This means the defending team wins the round more often than they otherwise would.
I also think Conquest has some imbalances, such as Arica Harbor Conquest. The Russian team has an easier time breaking out of a main base pin than the American team. The Americans have to make their way down a hill with little cover while everyone on the map has a shot at them. Russians can sneak out of the main easier and flank a capture point. This is especially apparent in 8v8 Tournament play.
Also, another thing to consider about level design is whether or not it promotes entertaining gameplay. Even if a Rush map is balanced, and the teams are comparable skill-wise, there are points on maps where it feels as though without fail, if you are the attacker at that stage you might as well be a Russian soldier charging the Germans in "Enemy at the Gates". And if you are the defender, you are waiting around trying to find a balance between scavenging for a single kill, not getting sniped by some guy 3 miles behind the frontlines who couldn't care less about winning, and blowing up random stuff in a feeble attempt to combat the boringness of an over-conservative defender.