Um......... can you really catch Articuno, Zapdos, and Moltes in this game... I don't think so I mean I don't recall that in the jap version, that's what FireRed/LeafGreen are for.
I think this game basically makes Coliseum completely useless, now all pokemon can be caught with in the GBA realm. Though coliseum is still fun to play.
The coolest thing is the ability to get Cyndaquil/Chikorita/Totodile, along with multiple other GS pokemon. Plus there is a whole new island added for MEW, obviously a nintendo event, so you'll have to cheat to get there for now. Plus the coolest thing which I haven't noticed been mentioned yet, is being able to rebattle gym leaders after you have beaten them. Each time their pokemon are stonger, and they even get new ones.
Lugia and Ho-oH are in this game, which completes the legendary bird collection, however it seems they are only accesible through a nintendo even just like MEW/CELEBI/DEOXYS/JIRACHI, only jirachi's event was the pre-order disc that came with Coliseum. Celebi's sprite was found in one game, however he does not appear on any island or anything like that, so it would probably come out with Pokemon DX just like Jirachi came with Coliseum. Deoxys and Mew will be available through tickets it seems, as they have something you must to catch them, ie. solve a puzzle for Deoxys to appear, and Catch Mew before you can battle it and catch it (do not get confused on that one).
If you want to know how you end up getting Cyndaquil/Chikorita/Totodile, you can ask, but you should probably find out yourself since you'll probably find out on your own anyway. That's really all I can think of that matters for now. Well other than battle frontie, which has an area with high level pokemon that randomly appear, which you can't catch. Ultimately the highest level semi-wild(meaning they are in a park, obviously they are not entirely wild) pokemon I've seen in a pokemon game, without using that glitch in red/blue.