Yes, Platinum follows the same trend that Crystal and Emerald before it followed, which is that the third game in the series takes what the first two presented, and improved on it heavily. Aside from the higher availability of legendaries, there's also other added features, like Gym Leader rematches and Battle Frontier (which was only a Battle Tower in D/P). Not to mention the fact that Platinum made the environment of the world change by adding snow on the ground in various places, which also made the male and female protagonists change into winter-wear. o.o
I also want to point out something more technical. Platinum obviously has more features, but there's also an issue with how slow certain actions happen in Diamond/Pearl, especially in battle. If you're used to the speed of battles in HeartGold, you'll definitely notice a heavy lag in the battle speed of D/P. Platinum doesn't suffer from this, though. x_x