Horford's signing did almost nothing to address the 4 major areas of concern from last years team (in no particular order):
1. Shot blocking
2. Rebounding
3. #1 scoring option
4. consistent/efficient 3 point shooting
You see that is why I've pretty consistently said that while Horford is a significantly better player than Sullinger, I don't see his signing as adding much to win the win column of Boston. Horford's strengths are already strengths on the team and his weaknesses unfortunately are weaknesses on the team.
I disagree with 1, 2, and 4 (kind of).
1) With our perimeter and overall team D, I really don't think "shot blocking" is as big of an issue as you make it out to be. Horford is by far our best blocking big, yet we were still a number 4 defense last year without him. I think shot blocking is overrated, especially when you have a strong, cohesive perimeter and team D to overcompensate for it.
2) The rebounding issue is overblown, too. I know it's only two games in the preseason, but Horford has owned the boards so far without an elite rebounder next to him. Amir is also a serviceable rebounder himself. We won't be a top rebounding team, but I don't think it's going to really hurt or cripple us anyways. Plus, when we trade for Boogie this will be a non-issue.

3) Agree on this.
4) Somewhat agree on this, but adding Horford is big for us in this regard. He's a much better floor-spacer than Sully, which will open up so much more space for IT. So while Horford isn't an elite three point shooter, though he is still probably top 10 for starters at the big position from three point range, he does add additional floor spacing at the big position that just wasn't there last year in our starting lineup, which is huge for IT.