I agree that there is a bit too much underestimating going on, and that Frye and Love will more than likely be the X factors, from a 3-point shooting and rebounding standpoint, respectively.
Regarding your first point, it's absolutely true that Cleveland has more offensive options than last year. Unfortunately, these additional options are defensive sieves, so a balancing act will be necessary, and not something a rookie coach is traditionally good at. To answer another poster above, this is why the Cavs aren't automatically that much better with the addition of better offensive options. You can't put seven players on the court at once, so it's going to be necessary to balance these better scorers, who will be taking touches away from LeBron and are generally dreadful defensively, with better defensive players that will make LeBron the go-to scorer. Balancing between these schemes is going to be difficult for Lue, who isn't above rookie coaching foibles.
Regarding your second point, I'll note that OKC took those three games because they weren't primarily running Durant and Westbrook isolations. When their ball movement was on point and Adams-Ibaka-Waiters-Roberson were contributing, they were off to the races. When they fell into their typical hero ball tendencies, they collapsed. In Game 6, for instance, the Thunder had 13 possessions in the last 5 or so minutes. They only passed the ball more than once on one of those possessions. Shit like that is why they lost 4. Decent coaching that kept them away from iso-ball is why they took those 3.