It has great animation, good characters, and a pretty good plot too (Romeo and Juliet - With Mecha!). All the action sequences are great too, and they redesigned some of the more goofy-looking designs from original Gundam into more plausible designs too. 
Its setting is kind of like 'Vietnam war meets gundam', although not quite as bleak as most 'nam war movies, but that's not necessarily a bad thing.

[QUOTE]Another thing to think of is how they will all fit in combat. As using wing and seed for examples, wing's gundams are just made of a very special alloy, which is why they are so durable and strong, in seed, the gundams all seem to have phase shields, wing uses bullets for projectiles, seed uses energy. (sorry if I am wrong, it has been a while since I watched seed)[/QUOTE]

I'd also say this is something to point out too - in Gundam Wing, the 5 gundams tend to 'steamroller' most of the grunt suits and opposition they come across short of the Tallgeese and the enemies in the latter half of the show, whereas in most UC and a handful of the other series, the grunts can put up a decent fight, and there's more of a risk element involved, and the sides are more evenly balanced. 
The phase-shift armour in Seed is a good example of tech needing to be balanced too, and the setting needing a unified background - there weren't any N-Jammers, the devices in the Seed universe that stopped nuclear reactors from working, in the other settings, just as Minovsky particles were only in the UC setting. 
Also, if you get G-Gundam in there, then there's even more weird stuff to consider.