These two observations are directly related, and pretty much explain the success of the 1970's Steelers, the success of the current Seahawks, and the struggles of the current Steelers: an advanced ability to spot and develop talent.
Only the Steelers saw how good Stallworth was; after taking Swann in the 1st, Noll was concerned that Stallworth would not be there in the 4th. (The Steelers had traded away their 3rd round pick.) But Bill Nunn knew that other teams completely underestimated Stallworth, and the defensive staff was adamant that the team not go offense with both of the first 2 picks. The Steelers scouting allowed the Steelers to draft another guy in the 2nd round that other teams basically ignored - Jack Lambert.
So the Steelers superior scouting allowed them to get Swann, Lambert and Stallworth with the first 3 picks.
Today, the Seahawks get Russell Wilson in the 3rd round, Richard Sherman in the 5th round, Kam Chancellor in the 5th round, Max Unger in the 2nd round, and two dominant players with the 6th and 14th picks in 2010 (Russell Okung and Earl Thomas). Their scouting is finding All-Pro players.
As you point out, of late, the Steelers' scouting of late has brought them a superb 6th round WR (Brown), a very good LT in the 7th round (Beachum), but a lot of mediocrity in the 1st round (Rashard Mendenhall, Ziggy Hood, Jarvis Jones). Cannot fail with 1st round selections so frequently and expect to have the talent to compete in the NFL.