It's really only the most important year if you're planning to apply to grad schools during your 4th year. That's because if you apply during your 4th year you won't yet have 4th year work for your portfolio, and you won't yet have 4th year grades in your GPA.
In any other situation I feel that your 4th year will be more important - in terms of the level of completion expected in your work and its importance in your portfolio - both for applying to grad school and for jobs.

Junior year for me was a year when I felt pretty comfortable in the department, knew all the faculty by then and generally what was expected of us, etc. The projects were a little more developed. There seemed to be a little less room for outlandish experimental projects that year - I mean there was still room for creativity but it was expected to result in a more realistic/practical project than in sophomore year.

I wouldn't get too stressed out about this year being more important than others. No reason to pressure yourself any more than last year.

Thanks eeayeeayo- however I would say that as of now I definitely plan on (trying to at least) going straight on to grad school after 4th year finishes... I've observed that doing so has become less popular in recent years but it seems to still be the most popular course of action and one I would like to take (for time concerns, and eagerness to continue with my 4+2). ..

But you are right, pressure only makes projects worse. I've found that the more (unneeded) pressure and anxiety I put on myself, the worse my results seemed to be.

Is it unrealistic for me to attempt to go straight into my M.Arch?

I remember my prof said in 2nd semester of 3rd year that this was when all talented students would really take off and leave everyone else in the dust. he was right, it happened and thanks to me i wasn't one of the ones eating dust.

I suppose it varies from student to student, but I don't agree with your professor, domestic. When I was in school, the talented people stood out right from the beginning. They shook the high school overachievers...and those who wanted to do better understood so much there and then. Looking back, my second and fourth years were my strongest, with the fourth year clearly the best. My portfolio earned me one of 22 spots in the ivy I had applied to for grad school.

That said, I believe that 85% of what I learned in school came in grad school.

as for 3rd year ... i think it is where you really start to put everything together. for different people it hits at different times. but 3rd year you have understood structures...MEP, materials, etc. with this it starts to be about so much more than just a pretty building!

as for going straight through... i urge you to wait. there really are 2 reasons. 1st a break will give you a renewed desire...and 2nd more of an understanding of what you need to still learn. as of now you would just be doing what were before... find out what you really want to learn...by getting out there for a year or so.