A. Most, if not all of the pokemon are not even close to similar, even among the same types. Movesets and stats have significant impact.
B. You don't go from city to city in the same fashion. I could go into the various new HMs and bicycling tricks needed to do so, but I think you can do that research for yourself.
C. Getting the badges is merely the first part of the game, unlike Red/Blue, where it WAS the game. Catching all 151 and beating the Elite Four was all you had to do. Beating the Elite Four is nothing more than good experience and training for the various events and quests you do afterwards. (Such as going after Cyrus in Platinum)
D. If you think there's no complexity to the battling system, you haven't heard of Effort Values. It's what makes your lvl 99 mewtwo absolutely terrible compared to a friend's lvl 99 mewtwo. It takes a LOT of research and bookkeeping to make your team truly the "best", and not simply "enough to beat the Elite Four".
E. In addition, there's other mechanics such as abilities and weather that have significant impact on how a battle will progress.
And what do you mean by "multiplayer aspect more than an SP game"? It has battling, trading, and if you're really bored, various minigames. I don't know what else you'd do?