Whatever Masahiro Sakurai and his team do, they will never please all Super Smash Bros. fans, and that hasn't changed with the Wii U or Nintendo 3DS entries. That's to be expected in a fighting series with complex mechanics and an ever-growing cast and feature-set.
Each title brings accusations of balancing problems, of course, which Nintendo has been able to tackle in the latest entries through software updates. It's very much up to the discretion and judgement of Sakurai-san, of course, and he's previously taken the opportunity to puncture popular misconceptions with behind-the-scenes data; one example was highlighting that rather than newcomer Little Mac being overpowered, poor implementation from many players made him the least successful character in the 3DS entry.
As mentioned above, though, not everyone will always be happy. A common target for criticism from particularly serious players in the current entry is Diddy Kong, with some arguing that the cheeky little ape is the biggest problem in the latest games. It's his aerial combo attacks that are the biggest offender, it seems, which are problematic not due to their existence but the level of damage they cause.
Kotaku has spoken to a number of competitive players to get their perspective on the character. The 'down-throw up-air combo' is deadly as it "works on everyone, can combo almost at any percent, and can even KO your opponent around 100+%", in the words of pro player Rich King ("Keitaro").
Max Ketchum ("Doom") provided plenty of detail, while maintaining that a slight adjustment to damage levels would suffice to ease player's suffering against those utilising Diddy.
Let me preface this by saying that, in my opinion, IF DIDDY KONG'S UP AIR (up + A in the air) HAD ABOUT 25% LESS KO POWER, NOTHING ABOUT HIM WOULD BE UNBALANCED. He would still be a very powerful character, but powerful characters are hype and great to watch/play—overpowered ones quickly grow tiresome and become degenerative to the game's competitive value.
Since a few weeks into Smash 3DS's release, people started noticing Diddy was a very powerful character. [Players like] Leffen (Sweden), Jtails (New York), Edge (Japan), Cyve (Germany), and Zinoto (Michigan) [played as Diddy Kong and] were emerging with impressive tournament finishes and gaining recognition from people who had played with them online. By the time the Wii U version was released, many pro players attending a launch invitational tournament hosted by Sky Williams in Southern California decided Diddy Kong was the ticket to victory because of his low learning curve and devastating potential.
In the competitive scene debate can be heated over whether the character should be excluded, while there are weaknesses - if you counter successfully to deprive Diddy of his jet-pack - to get on top; these countering tactics aren't easy, of course, otherwise this wouldn't be an issue.
Do you feel Diddy Kong is over-powered - OP - in the latest Smash Bros. entries, or fine as he is? Is there another character - or characters - that are bigger problems? Is this all over-the-top and not worth debating? Let us know what you think in the comments.