I think the main reason everybody is sleeping on the Ravens this off-season is they were downright awful last year. There were tons of injuries to very important pieces, and we'll have to wait and see if they can return to form after recovering.
For reference, they lost QB1, RB1, RB3, WR1, WR2, WR4, WR6, WR7, TE1, TE3/4 (and Dennis Pitta, who hasn't returned for 2 years, but had been TE1), C1, LT1. That's only the offense! The defense suffered a couple important ones as well (mainly Suggs), and some important depth. I couldn't even remember all of them, I had to look at a list and then give rough guesstimates about depth chart.
If most of those (or their replacements this year) can perform up to standards, the Ravens could be a very dangerous team, but there are too many variables to definitively say they'll be good or not. This year's draft picks look promising. As a Ravens fan, I wouldn't be surprised going 12-4, nor would I be surprised going 4-12 this year, just depending on recoveries and all that good stuff.