He is great at offense, but his defense is godawful.
Now, before I refute this remark, let me note that my concept of "defense" in Brawl and the SSB series may be different from yours. For clarification, I'm talking about passive-aggressive play, as there are few situations in this game where you can be ENTIRELY defensive.
In my experience with Ganon, he (like a majority of the Brawl cast, perhaps this is inspired by the new game mechanics) plays best when used "defensively." He's a momentum character, which means you need to watch out for that one opening and let your pseudo-combos roll forward from there. Because of his sluggishness on the ground and general startup lag on ground moves (dash attack and jab being the exceptions here), he lacks the ability to play aggressively for the match's entirety. However, he plays aggressively in short bursts, meaning short stretches of time in which Ganon can force the opponent to play at his pace. This is best accomplished by waiting for the right moment to do any of the following things (things which are generally avoidable if used carelessly/aggressively):
Side+B, Dash Attack, Dtilt, Ftilt (near ledge, ideally), Throws (Ganon's short dash and slow running speed make this a somewhat difficult feat, thusly it must be used carefully, i.e. shield grab), SH Autocancel Dair (a little slower than the rest of his options [startup wise], I tend to use this even more precariously as an opener, though it is still very viable, I find it to be better as a closer or followup to any of the former options), SH nair against taller characters, Bair
Actually, that's almost Ganon's entire moveset. My point is this: almost all of his moves will leave you in a (modestly) punishable position if you just throw them out left and right. To me, Ganon is a cautious character. I typically use side+b to get the momentum of his offense rolling, but getting there is an uphill climb which must be confronted carefully. Overall, I see why you might characterize him as "offensive," but I generally find him to be more of a momentum character than an all-out offense junky.
That said, here's my concept of Ganon's metagame:
Currently, the American Smash community has formed a general consensus that Ganon is the worst in the game. Verily, an appearance by Ganon at any large-scale tournament will shock that majority of its participants. As he appears more at tournaments (or as the current Ganons appear at more tournaments, as current sentiment about Ganon's playing ability does not bode well for the number of Ganon mainers), people will gradually abandon this conception of Ganondorf (and from there perhaps cast Falcon for "worst character in the game").
Once that's dealt with, players will generally find Ganon to be intimidating, as almost all of his attacks lead into viciously painful pseudo-combos or death at low percents. Barring matchups in which Ganon strikes a distinct disadvantage, players will generally approach Ganon as carefully as (I think) he should approach them. Even against ranged characters, careful approaches via air or powershielding will allow Ganon some room to force the opponent to play at HIS pace. The risk that a player runs in suffering just one opening hit from Ganon hopefully accomplishes this much. Again, I find this important to Ganon's metagame because, once he gets the ball rolling, there's a good chance that he can run an opponent into dangerously high percents or to death in a single good combo. Ganon's ability to set the pace of the match like this lets the momentum come more easily.
Side+B and shield pressure are also essential to establishing this sort of tone for the match. Used cleverly, these can basically force an opponent out of his or her comfort zone as far as shielding.
Each of these pieces of Ganon's metagame makes him at least playable, and these, combined with simple knowledge of Ganon's moveset, easily earn Ganon a "Get out of 'worst character in Brawl' place Free Card," but there are a few things that keep me from thinking that Ganon can ever feasibly WIN tournaments. He's definitely a competitor until the later rounds, but the fact of the matter is that the number of disadvantageous matchups for Ganon is significantly higher than that of most tournament-winning characters. He suffers most heavily in matchups where the opponent is squirrely (Squirtle, Pikachu, Luigi, Jiggly, Wario, even Falcon) and in matchups where the opponent can easily combo/punish characters simply because of their size (characters with epic sweetspots such as Jiggly, Luigi, and Falcon come to mind, Luigi and Jiggly moreso because they can feasibly "combo" on heavier characters who also take up a lot of space). Thusly, I find that Ganon shows promise until he's faced by any of his numerous disadvantageous matchups. Since the probability that Ganon will be put into difficult matchups is fairly high, I don't find it likely that a Ganon player will be able to endure an entire tournament bracket without facing some VERY difficult matches (that is, matches made harder by Ganon's personal shortcomings, not those caused by the skill of either player).
He's good. He's fun. He's epic. He's very good. But he just won't be able to finish tournaments. Ganon mains are best off practicing counterpick secondaries, as I don't see Ganon ever taking himself all the way through a tournament. He is most CERTAINLY a formidable force in Brawl, though, and shouldn't be as underestimated as he is. However, my belief is that he won't be able to endure an entire tournament until there is a revolutionary change in Brawl's or Ganon's metagame.