With regard to the level themes, I too was disappointed in how generic a significant number of the tracks felt (not that this is something entirely new to the series). MK DS had some tracks that really felt embedded in parts of actual Mario games (such as Delfino Plaza, the Tick-Tock Clock track and Airship Fortress), and satisfied both as racetracks and as creative adaptations of parts of the Mario series proper. There are a few nice moments in MK Wii (like the underwater part of Koopa cape), but too often the themes feel scarcely related to Mario at all (Maple Treeway, Grumble Volcano), or are just glib nods to the Mario franchise like a character name and a giant statue (Luigi and Daisy circuits). This isn't very important in the grand scheme of things, but it adds to the feeling of the game suffering from a lack of creative flair given its many similarities with other games in the series.
To Jeff's other point: my considerable experience with the GP mode (getting all the unlockables requires getting star ranks on almost every cup in every class) has made it very clear beyond any doubt that items are deployed against you with a far higher frequency in 150cc mode than in the lower classes, and this is the primary way in which it is has been made more difficult to succeed in that competition.
We're not talking red shells here either, but rather it's the activation of blue shells, lightning bolts and POW blocks that seems to get increased the most to impede your progress. Also, this increase in item intensity and frequency has been designed to be asymmetrical; if you are in first by a long distance then it is quite likely you will be pummelled with items until you are no longer in first, whereas if you're a few spots back of an AI player you're far less likely to benefit from them suffering the same fate.
So basically Nintendo have increased the difficulty by just changing a few statistical probabilities so that they're weighted against you, and when the consequences of this are felt, it is inevitably frustrating. This is why I desire an increase in difficulty that arises from the AI racers being superior racers rather than the arbitrary dialling-up of item activation. Also, if this were the case you would be motivated to find ways to improve your own performance. In MK Wii, success is a function of possessing sufficient perseverance to try your luck enough times that success is a statistical inevitability, and therefore does not appropriately incentivise improving your racing performance. I'm not exactly sure how this could be achieved beyond just "better AI" (having an item set designed specifically for 1player GPs could help), but that's Nintendo's job, not mine.
Online play throws the shortcomings of GP into stark relief, as the items are clearly not weighted against one player in particular. This makes even the more unpleasant ones far more tolerable, as they don't occur as frequently and you will generally benefit from them as often as you suffer. Human competitors also give an idea of how much better the game could be with smarter AI; online play is just so much more fun than grinding through the GPs.