Although I'm only a D1, but I have heard many horror stories about the second year from many D2s from more than one dental schools (Nova and Univ. of MN).
First year is tough for some because of the graduate level work transition and the heavy load of basic sciences on top of some preclinical lab courses.
But second year is where everything hits you. I believe that in general, all U.S. dental schools, during the second year, you still have some basic science courses to be completed. In addition, A LOT more preclinical courses (operative) and hours than the first year! Furthermore, second years have to worry about the NBDE Part I, either in December (at some schools) or in July (most schools). Also, some patient exposure maybe integrated by this time of the dental education.
I know for a fact that at Nova, our second semester second year is the WORST!!!
Each dental school is different, but similar in that you see patients more regularly third and fourth year while first and second year are all didactic and pre-clinical lab classes. So first and second year are spent learning all the Anatomy, Physio, Histo, Micro, path, etc with the med students in addition to learning how to cut cavity preps, make dentures, do root canals, do crowns on mannequins and plastic teeth. Third and fourth years are spent doing those same procedures, but on real patients with real needs. I can only give you my experience at Buffalo.
First and second year both suck. First year involved sitting in academic classes from 8 - 4 and then trooping to the library to sit some more and study till midnight. I hated the non-stop sitting and the studying. First semester, second year is the worst at my school b/c not only are we taking micro and path with the med students, but we also had many dental lecture classes. That's not all - the most time consuming part of this semester is the pre-clinic lab courses in operative, fixed, and removable. We were in school 8 - 5 every day, then you have to stay after to finish the lab projects they assign you (NO ONE ever finishes them in class, it is intentionally designed this way to give you more practice with your hands). After putting endless hours in lab, you also have to study for micro and path somewhere in there. You work every day, Saturday and Sunday included, and there just aren't any breaks. Often you can't even skip a class if you wanted a break b/c they give quizzes at 8 am sharp just so people won't skip.
Which one was worse? I can't decide. Now that boards are over, I'm trying to block both of them from my memory.
On the other hand though, I am now in third year and I have to say THIRD YEAR ROCKS! It is a good blend of classes and clinic and there are nights when I go home, eat dinner, watch TV, and fall asleep. We have class from 7:45 - 8:35 MWF, see patients from 9 - 4 MWF and sit in lecture classes from 8 - 4 TR. All of our classes are now dental related except for Pharm. Seeing patients is cool too b/c this is what I came to dental school for.
Third years at my school encourage the first and second years by telling them it gets better in third year.
Dental school is different from medical school. I lived with a med student during my first two years of dental school while she did her first two years of med school. She averaged 1 test per week (if that) and was in class from 9 - 2 (at the latest). Sometimes she skipped even those classes b/c she saw no need to go. Contrast that to my second year where we had at least 3 quizzes per week, two practicals, projects due all the time, and at least 1 major exam. I'm just telling it like it is - I did more work than her the first two years but this year, she is paying it back b/c her third year is hell compared to mine.
I think is also depends upon the individual. The one that I know likes second year much more than first year even though the former is much more time consuming. For that person, the increased work load in the lab helps to counter balance the lectures while the big exams supplemented by weekly quizes create less emotional stress than did the all or nothing mid terms and finals of the previous year.