Yes, it was definitely a let down. I remember when I was playing through Platinum, it had this strange epic feel to it. I was disappointed that HeartGold lacked that. And the levels are way too low in HeartGold, because they were too lazy to fix the problem where you could battle the 5th, 6th, and 7th Gym Leader in almost any order you want. And the Johto dex is extremely messy and horribly laid out, WHY ARE THERE SO MANY KANTO POKEMON. And the level progression is terrible. First of all, Whitney is only 2 levels higher than Bugsy, Chuck, Jasmine, and Pryce all have basically the same levels, meaning no level progression at all, ROUTE 43 HAS POKEMON AT LV15, ARE YOU SERIOUS, in the E4 rematch teams, Lance's Pokemon are +12 levels higher than Karen's, and Koga, Bruno and Karen have no level progression at all (they all top out at Lv64, while Lance jumps to a whopping Lv75)... THERE'S NO VS. SEEKER, AND THE PHONE REMATCH SYSTEM IS HORRIBLE
Overall, I think HG/SS is a mess. Platinum is not.