In the case of BC2 and Frostbite 2 vs MW2 or whatever CoD game that came out that year, I actually wouldn't be surprised if the character models themselves were also higher res. The more obvious differences between the two engines would be when it came to things like player count, map size, map interactivity (distruction and such) and all the various land and air vehicles in the game. You also have to take into account the different physics needed to make vehicles work in the game.
But this really isn't all that fair. CoD isn't trying to be Battlefield. You can't say "Look at all the destruction and vehicles in BF. It clearly has the better engine because of these things." Those engines are obviously made for different things. And sure, maybe BF's engine is more impressive because of the type of game Battlefield is, but then again I've never had CoD freeze up on me and that happens every time I play BC2 for more than 30 minutes, so there may be something to be said about BC2's Frostbite engine as well.
Character models have higher poly counts and they're taking advantage of better tools to make more detailed worlds.
Like I said, that's not really the same thing as the engine. It has more to do with engine optimization, not the engine at its core. This is evidenced by the fact that on the PC you can change your graphics quality to whatever you want on most games.