For me, the killer feature (aside from tiling vs overlapping) is that on my three monitors, I can switch virtual desktops independently on each monitor. After being able to do this, I don't understand why every window manager doesn't offer this. It's not uncommon with tiling WMs that have good multi-monitor support, but you really can't find it in traditional WMs. There's an unmaintained patch of openbox--openbox-multihead--that added the feature, and there's an "experimental" hybrid window manager written in Go (by the same author of the openbox patch), but no mainstream window managers appear to have the feature. Apparently it goes against a requirement/assumption in the EWMH spec and breaks existing pagers. But that's okay, because I don't use a pager.
Anyway, it really is critical to how I work now. I can keep a set of windows on my right monitor, and flip between virtual desktops on my center monitor without the right monitor changing at all. (And no, using the "sticky" feature of other WMs doesn't work for me because sometimes I do want to flip to a different virtual desktop on the right monitor and have that set of windows for whatever context I'm switching to). Along the same lines, the pertag patch is part of making dwm the perfect windows manager for me.
(If you're interested in the particular set of patches I use and my config, I maintain it at https://github.com/racingmars/dwm)