Tough to grade someone on something that you know far less about than the person you are grading. The OP is on point that there is so much that fans don't know that goes into the daily work of being an NBA GM.
My perceptions are the following:
1. Danny is smart. I suspect that his basketball knowledge and GM-ing/business knowledge is comparable to the best GMs in the NBA.
2. Danny is crafty and creative. I think he tries to work all angles, even if they don't work out.
3. Danny is a hard worker. He impresses me as someone who is constantly thinking about his job, and constantly working at making the Celtics better -- sometimes for the short-term and sometimes for the long-term.
4. Danny is flexible, resilient, and realistic. He can make mistakes and bounce back. He is able to go down a road, see it's not working, and make the best of the situation. He can deal with the reality of the marketplace even when the results may be dispappointing, unexpected or unpopular.
5. Danny is not afraid to take educated risks.
It also should be noted that winning an NBA title is extremely difficult. Since 1980 (35 years), only 9 franchises have won an NBA title. By my count, 17 of the 30 current NBA franchises have never won a title in their team's history (nearly all of these teams have not seen an NBA Finals either). Since Danny became the C's GM, Celtics have been to two Finals and won one.
It should also be noted there are two distinct avantages that occur in the NBA:
1. Warm weather franchises have a huge advantage in free agency. The LA Lakers in particular, as both a warm weather and flagship franchise, need only to be smart enough to free cap space in order to rebuild (they'll always be back in the thick of things within 5 years).
2. The best NBA players win championships. Franchises lucky enough to draft or retain services of MJ, Jabbar, Bird, Magic, Hakeem, Duncan, Shaq and Kobe have accounted for 27 of the last 35 championships.
Danny is doing great.