I'm a Pats fan living in Baltimore who will also be attending the game. I've been to 4 Ravens games over the past few seasons, and how the crowd will be really depends on where you are sitting. The upper deck has a reputation for being tough on opposing fans, especially in the corners and end zone sections. In the lower deck, sections 144 through 148 are very rough on opposing team fans but as long as you're courteous, the not-so-drunk people should be pretty friendly. Keep in mind that as Pats fans, we're coming in with the bad reputation of the Red Sox fans who inundate Camden Yards every baseball season (Red Sox fans are the most hated subset of opposing team fans in Baltimore. Yankee fans don't hold anything on how rude the Red Sox fans at Camden Yards tend to be). Be nice and you should be fine.
Speaking of this topic, are there any Pats fans who will be tailgating and wouldn't mind sharing some good times with a couple of fellow fans?