When I first started with recording connection, one of the first things my mentor (Joe Haze) taught me was about reverb. It's been my very favorite effect since.
I guess I've always known was reverb was, I just didn't know what it was called. Of course, in a close second is delay, which at first I didn't understand the difference, but this chapter does a good job explaining what the difference is.
Those two effects, for me personally, totally change the emotional aspect of the song, which I love way more than I should. Actually on my mixes, I put more reverb than I should probably put.
For me, time based effects with the exception of volume make the most difference to me in how the song feels.