Kevin Durant is a very athletic 6'9", 20 years old SF. Ben Gordon is a 6'3", 26 year old SG. Kevin Durant has the perfect length (7'5" wingspan) for an NBA SF. He is longer and more athletic than most SFs, which along with his amazing jump shot makes him almost unguardable. Like McGrady, he is so much longer and athletic than the players who guard him that he can get off his shot almost anytime he wants.
Durant is a better scorer. Durant's points per shot this year (1.35) was better than Gordon's career best (1.31 in '06-'07') and Durant's career PPS (1.27) is better than Gordon's (1.24) even though we're comparing a player who entered the NBA as a 19-year-old and who has only played two seasons to a player who entered the NBA as a 21-year-old and has played five seasons. Not only is Durant a better scorer, but he is just scratching the surface of his potential.
Durant's size also makes him a better rebounder. Gordon averages 3.0 RPG for his career and had a career-best 3.4 RPG this season. Durant averaged 4.4 RPG while playing SG his rookie season, and that average jumped to 6.5 RPG this season (I know I've calculated this somewhere before, but he averaged right around 7 RPG when PJ was fired and Brooks moved him from SG to SF). Durant was third in the league among SFs in RPG (behind LeBron and Gerald Wallace, and Wallace also plays minutes at PF); Gordon was 16th among SGs.
Durant has all of the tools to be a good defender. He is very young and has a long way to go. He just needs to get a little stronger. He is never going to be a bodybuilder, but neither is Tayshaun Prince, and he turned into a very good defender, largely because of his freakish length (BTW Durant has a slightly longer winspan than Prince). For that matter, LeBron was also very undisciplined defensively at Durant's age, and only recently made the full commitment to defense. Again, Durant is 20 years old and nowhere near his peak as a player, whereas Gordon isn't going to get significantly better: he's an undersized SG who will always be too small to guard NBA SGs, and he doesn't do anything but score. Gordon is a very good player, but Durant is on another level. There isn't a player in the league besides LeBron who matched Durant's offensive production as a 20-year-old.
After two seasons:
LeBron-- 24.1 PPG (1.21 PPS), 6.4 RPG, 6.6 APG, 0.69 BPG
Durant-- 22.7 PPG, (1.27 PPS), 5.4 RPG, 2.6 APG, 0.8 BPG
Durant isn't quite LeBron (especially as a distributor, not to mention strength), and I don't expect him to be, but there isn't any other player in the league I would take over Durant right now. Players typically don't come close to peaking as 20-year-olds. Durant significantly improved from season 1 to season 2, and he's going to be the best scorer in the league in his prime.