More like Melee 2.0, which isn't necessarily a bad thing
That is the problem with Brawl, it's basically an update to Melee. Put in a Smash Ball, Subspace Emissary, and 10 more characters, and boom, that's Brawl. Of course, there is the online mode, but... well... let's just say it isn't "up to par". I mean the controls are exactly the same, in the first few days the game was out people would ONLY play on Melee stages online for some godforsaken reason. It just seemed more and more like a beefed up version of Melee.
Now, Melee was a fantastic game, in my opinion it WAS the game to have on the GameCube (excluding Resident Evil 4), so the above paragraph isn't really a bad thing, especially for newcomers to the Super Smash Bros. series. They basically mastered the controls in Melee, so I understand their reluctance to deviate from that.
One of the most anticipated things in the game (other than online) was the Subspace Emissary. Subspace Emissary took the place of the Adventure mode that was in Melee, and has some sort of a story to it. It is much different than the majority of the game, as it is more of a platformer than a fighting game, other than boss fights which returned to the fighting genre. This mode is okay, it adds a lot more time than Adventure mode would, as it takes about 8-10 hours to complete depending on difficulty. Of course, the final stage takes forever, and after about 20 minutes you'll ask yourself when it's going to end. The one great thing about the Subspace Emissary is that you can unlock all the characters without playing a single VS match. You have to beat it before you can get certain characters, but you can unlock every character, which is better than playing 700 VS matches just so you can unlock someone.
Now, this game will last you a long time. The Wii doesn't have an achievement system, but there is a area of the game where you can see how to unlock certain maps, songs, trophies, and create-a-map parts. This will keep you going, and going, and going, like the Energizer bunny. You can easily play this for over 100 hours, but I'm not a perfectionist, so I didn't spend quite that much time. Still, it'll keep you busy for a very, VERY long time.
Gameplay:9
Single-player: 7
Online:6
Lasting appeal: 10
Overall (not an average): 8.5