IMO, Flacco is a good quarterback, not a great quarterback ... but certainly good enough to not stop a team's Super Bowl aspirations. His deep ball is undeniable, his poise is rock solid ... but there's just something about Flacco, to me, that just cries out borderline top 10 QB, but not a franchise QB whatsoever.
Maybe it's the Brady thing. Some people criticize Brady because a lot of his completions are 5 yards or so. Nothing spectacular, but his completion percentage and winning percentage are sexier than Flacco's.
IMO, Flacco has to embrace more check downs.