Major issues from last year: interior defense (Amir is good but can't play much), shooting/spacing, scoring in the half court, and rebounding.
Interior defense: Solved. Horford is a very good rim protector, and between him and Amir, we can have a rim protector on the floor for 48 minutes.
Shooting/spacing: Helped. Horford offers shooting at the 5, which should do wonders for our spacing. IT will have a lot more room to work, making him even more deadly. Gerald Green might offer some additional shooting off the bench, and we can hope that Marcus will no longer be the worst three point shooter in history.
Scoring in the half court: Helped immensely. Horford is an outstanding passer, elite PnR finisher, elite screener, excellent post player, and excellent mid-range shooter. We finally have a second legit scoring threat. We won't have an elite half court offense, but we'll no longer be painful to watch.
Rebounding: ?. We lost our best rebounder, right? Technically, yes, but we did actually rebound better with Sully off the court. A similar phenomenon exists in Milwaukee, where Greg Monroe seems like a great rebounder, but the Bucks actually rebound better with him on the bench. Compound the loss of our "best" rebounder with the addition of a "poor" rebounder in Horford (some have suggested that Horford's declining rebounding numbers are due to the Hawks scheme and presence of Millsap), and one would think that rebounding will be a massive issue. I'm not sure we'll be as bad as some fear, but rebounding will likely be a weakness.
The one major weakness remaining seems to be rebounding, and we'll likely still have some problems scoring in crunch time, but we addressed a lot of our weaknesses.