Some minorish Platinum tweaks which haven't gotten much attention

[bluaki]

Yay for Serebii: http://serebii.net/platinum/changes.shtml

The ones there that struck me as interesting which I haven't heard of before:
If you inadvertantly knock out a Legendary Pokémon, they have a chance of re-appearing later on in the game. (Ever since I first played Blue version, I've questioned why "KOing" a legendary essentially kills it, and the possibility of completely losing the legendary has annoyed me)
The Pokémon back-sprites are now animated.
Many Pokémon owned by trainers have PokéBall Seals attached to them.
Every area has received a graphical update
The lighting within Forests & Caves have been updated
The order you battle the gyms in has changed.
The speed while Surfing has been dramtically increased. (I've seen a number of posts in the past complaining about the slow surf speed in Diamond... this'll be a pleasant tweak)
The Graphical User Interface has changed to reflect the area that you are battling in (I saw that grey overflowing HP bar, but I didn't think it changes in different areas)
In Resort Area, you can find Magikarp from Levels 1 up to Level 100 (...I really don't see the point of this, but the pure randomness of stumbling upon a lv100 Magikarp in the wild amuses me)

And, a few other things that might be of interest to the rest of you, although I've seen them mentioned before:
The Sinnoh Dex has got a further 59 Pokémon, including all the non-legendary new Pokémon introduced in Diamond & Pearl.
Tons of Pokemon selections for wild, trainers, GBA-insertion, etc. have changed to accomodate for the expanded Sinnoh Dex
After obtaining the National Pokédex, Articuno, Zapdos & Moltres start to roam Sinnoh.
Move tutors
Every Pokémon now has foreign Pokédex entries (I'll like this one quite a bit--maybe obsess to the point where I try to get all 493 entries in both English and Japanese since I have a Japanese Diamond version)
The Elite Four is rebattleable at higher levels
Poketch has a back button
A few buildings have been changed, notably the Galactic buildings and a couple gyms.

[Macawmoses]

We've discussed a lot of this.

I'm glad surf got sped up, I'm glad the E4 progresses to higher levels (as does your Rival, up to lvl 85), and I'm glad for some cosmetic things and legendary rematches.

As for other things, Regigas will be available at Lvl 1 - making it the lowest leveled wild Pokemon....with Magikarp being the highest leveled (100, as you pointed out).

In general, Platinum has received numerous changes, and I am glad for it :)

[Nayrman]

Some of those changes are indeed pretty cool. Although I only get 1 per generation, usually the first two games. Just a habit of mine, since the only third one I bought after having the origional 2 was Yellow, only cuz it was based off the show.