@frozen joe flacco fan
I think if we're just grading the draft, it deserves a B. If we're grading the whole offseason, then a different grade might apply.
You can't realistically fill all your holes in one draft. They made each pick count until the Clark pick, and that was in the 6th round. We needed multiple pass-rushers, multiple O-Linemen, and CB depth (or possibly a better starter than Carr, who is old and declining). Replace any of the picks that were made with someone else and you have a hole or decreased depth somewhere else.
Also, Siragusa was introduced during the post-draft presser as a center/guard, for whatever that's worth. Perhaps he can become that Center you're looking for. In the meantime, he can play LG, which is just as big of a hole.
Anyway, FA isn't over yet. They'll probably add a veteran WR and OL in free-agency before TC starts, maybe even Boldin or Mangold. They also could trade for someone or pick up a cap casualty before the season begins.