When I was accepted into medical school, many people congratulated me with a "dude, you're leaving a good job to go back to school, med school's tough, why would you bother?!"
Friends who had graduated from engineering like I did responded a little differently, "dude, you're leaving a good job to go back to school, oh well, med school can't be that much tougher than engineering."
Having been through 1.5 years of medicine, I can certainly say that the engineers were very wrong. Conceptually, I don't think any single topic covered thus far in my medical education is any more difficult than the topics covered in electrical engineering. In fact, I would argue that courses on quantum mechanics and nanotechnology were probably more difficult to understand than anything I've been taught in med school. However, the major difference is volume of information. A single week in med school seems like at least half of an undergrad course. We have to read, understand, and memorize a ridiculous amount of information.
Which brings me to another difference between engineering and medicine. Engineering required very little memorization. For the most part, in engineering, you only have to memorize first principles and then you can usually derive any other equation you need. There is no way to derive the cranial nerves and their functions, nor is there any way to derive the different pathologies responsible for papulosquamous eruptions...there's no way around it, you have to memorize in medicine!
The workload in engineering is above and beyond that seen in most other university programs; however, the workload is much, much greater in medicine than even engineering. The only consolation med students have is that most schools grade on a Pass/Fail system, as opposed to a letter grade system. So for the most part,in med school, getting a 60% is as good as getting a 90% on an exam (not from the patient's point of view obviously), you don't have to worry about getting A's anymore.
The last difference that became very apparent to me last month is the difference in exams. My engineering exams were almost entirely math based; I don't remember ever seeing multiple choice on any of my exams. Med exams are entirely multiple choice, and there are a LOT more questions asked, which makes sense considering the significantly greater volume of information taught. After completing an engineering exam, most students have a good idea about how they did in that exam. After completing a med exam, most students have no clue how they did in that exam! A number of my classmates have seriously believed that they failed an exam, only to later find out that they aced it.