It is all the rational choice theory based econometrics stuff (which was everything that was taught).
Basically the assumptions they make are so far away from the real world that any conclusions they draw are neccesarily wrong... for instance, they assume that everyone is a rational, utility maximising consumer who has perfect knowledge (ie: knows all of the sellers in the marketplace and what price they sell at, and have equal access to all the sellers) and makes choices based purely on a cost-benefit analysis of price.
So stuff like the effect of advertising, the fact that no-one has perfect knowledge or is completely rational are totally ignored. There's no real interest in talking to psychologists about how people actually behave, because they have their assumption and all theories are built from that.
Then there are things they say that stem from the above, like every economic transaction that takes place is a positive sum gain and maximises the utility for both actors, on the basis that a rational consumer will only take part in a transaction that maximises their utility and so whatever takes place must maximise the utility for both parties, or they'd do something else with their money/product/time.
This extends for instance to employment whereby they think if I take poverty line wages because it's all that is on offer that somehow maximises utility for both me (because my only other option is to starve, clearly worse) and the employer (who gets to pay me fuck all). Completely ignores power relationships and the obvious factor that in the real world people make choices that they are effectively forced to make, not freely.
This then leads to the (logical, given the axioms) conclusion that in order to make the world the best place possible we should make sure there are as many transactions taking place as we can, since every transaction maximises utility. Usually this means taking away regulations. Total ignorance of how power relationships work.
Another example of this kind of thing is "perfect competition" which is something that cannot take place in the real world because of the conditions that are required for it - no barriers to entry for new businesses for instance, impossible, you can make barriers small but never take them away completely, every business needs some start up capital either in the form of cash or time at the very least. Also says that there are enough buyers and sellers in the market place that no-one has the power to set prices like in a monopoly, and that all buyers and sellers have perfect knowledge.
Now as an intellectual exercise thinking about different types of market that is fine, but for some reason they seem to think that perfect competition could actually exist, mostly if it wasn;'t for that damn government placing regulations on business (never mind that business tends towards cartels & monopolies when you look at the actual real world, because the theory of competition means that it should go towards perfect competition, even if it never quite gets there).
Should say that the above stuff does not constitute "economics", it's just one paradigm of economics - but it's totally dominant, especially in the realm of public policy.