What I wanted to ask you: can you really classify 'good' and 'bad' art?
I mean (as you are the pro) are there widely accepted rules and fundamentals how things have to look to be 'good' art?
There are some general issues about all kinds of commercial graphics and any game asset can be evaluated based on these, yes. It is generally independent of the style used
These include stuff like anatomy, balance, harmony, silhouette (including negative space, ie. where you don't have stuff in the forms), colors, relation of low, mid and high frequency detail (both in textures and form) and so on.
There are both age-old 'rules' and new scientific research on composition, ideal anatomy, complementer colors, the golden mean. All these can and should be applied to everything, but one also needs to know how to use them. Sometimes we have to go against the rules to get the desired results, especially in game art when a level has to set the mood, a monster has to terrify or some NPC needs to be sympathetic. Unfortunately it's also quite common to overdo things, for example make every female person a playmate...
For example Uncharted's Emily was a welcome change with her almost everyday look... and I also kinda think that most of the characters in Gears (particularly the humans) are not as good as people usually believe, but the insane amount of well developed details still manages to sell them.
It's a bit hard to fully explain this, even harder to precisely define... a lot of how you evaluate artwork is based on experience. For the lack of a better word, the eye or the sight of someone has to be developed through years of practice to make it a conscious process, so that the person can actually use it as a skill.
Our art director with a traditional art background and over 10 years of previous experience in an ad agency can look at something and immediately tell what's good, and what needs to be changed and how to make it look better.
On the other hand the average person can never tell these things, not even the difference; but they still have an instinctive feel of the general quality.
I'm actually a firm believer that there's good and bad in terms of general quality in almost every kind of art. Some people wouldn't even consider talking about stuff like good pop or heavy metal music, or good popcorn movies, and so on... but I don't really think in terms of 'there's only high art' or whatever you'd like to call it.
So is your rating 'HALO3 artwork is weak' your personal subjective opinion or your opinion as a professional (maybe it is hard to answer as long as you are not schizophrenic
)
There are objective issues with some of Halo's art assets in my opinion, as professional as it can be. For example practically everyone agrees that their human characters need work, but I would rather not get into more details about the rest, as it'd require far too many explanations