Here's a listing of the Offensive and Defensive Efficiency (per ESPN) of the Conference Champions for the last 10 years:
I also listed the best player for each team in parentheses.
15-16: GSW - 1st, 4th (Curry) .... CLE - 4th, 10th (LeBron)
14-15: GSW - 2nd, 1st (Curry) .... CLE - 4th, 20th (LeBron)
13-14: SAS - 6th, 4th (Kawhi) .... MIA - 2nd, 11th (LeBron)
12-13: SAS - 7th, 3rd (Duncan).... MIA - 1st, 7th (LeBron)
11-12: OKC - 2nd, 9th (Durant) .... MIA - 6th, 4th (LeBron)
10-11: DAL - 8th, 7th (Dirk) .... MIA - 3rd, 5th (LeBron)
09-10: LAL - 11th, 5th (Kobe) .... BOS - 13th, 5th (Garnett)
08-09: LAL - 3rd, 5th (Kobe) .... ORL - 8th, 1st (Howard)
07-08: LAL - 4th, 9th (Kobe) .... BOS - 12th, 1st (Garnett)
06-07: SAS - 5th, 2nd (Duncan) .... CLE - 17th, 4th (LeBron)
Boston Celtics this year: 8th, 11th (Isaiah Thomas)
Of the teams listed, I guess the Celts are most comparable to the 2010-2011 Mavericks. Like the Mavericks, the Celts are well above average in both offense and defense, but aren't top 5 in either category.
The other major similarity I see is that both teams relied a great deal on one elite offensive superstar to carry the team's offense, while the rest of the pieces picked up the slack in other areas. However, Dirk had won an MVP award in the past and arguably reached a higher level in the playoffs than he ever did in the regular season. Thomas, by comparison, is playing better in this regular season than he ever has in the past.
Unlike almost all of the other teams on the list, however, the Celts have no track record prior to this season of being a top team with the current core group. The Mavericks had made a Finals back in 06 and won 67 games in 07. You could say Cleveland was more or less in that position in 07, though they had at least won 50 games and a playoff series the year before.
Overall, I'd say it's a little bit of a stretch to see this Celts team making a Finals, but they do have one transcendent offensive talent in Thomas, and teams with greater flaws have made the Finals before.