Chrome looks about the same everywhere, emacs is customized within itself and I turn off all the window dressing on the terminals I use (no menus, no scrollbar, nada). Since that sums up 99% of my typical day to day computer usage there's no visual downside to xmonad for me. What looks even better than customized widgets (and I've tweaked emacs theme and the terminal settings to my taste long ago) are nice layouts that arrange all my windows in more or less logical ways automatically. Few things look as bad to me as a mess of randomly sized windows partially overlapping each other!

Could you link an example? Some tiling window manager users just don't care much about appearances (my general approach to window decoration is to get rid of it), but it should be possible to approximate the appearance of windows under any desktop environment via configuration tweaking (eg Firefox themes, font settings, GTK/Qt themes).

You are likely to find that a stand-alone WM requires more manual configuration than a DE. Most DEs have more-or-less unified settings managers, but those settings are really for many disparate components of the environment that a WM won't even try to control. It might be possible to recover some convenience by using a third-party settings manager like the previously mentioned lxappearance (I find the Arch Linux wiki a useful resource in this regard even though I don't run Arch).

Alternatively, you can use xmonad with Xfce (or another DE). That's what I did before completely switching to xmonad and it does make settings management easier.

In my experience, window manager authors don't usually care if some other program looks bad. There's no real unity, and their attitude is just "not my program, not my problem".

It's the correct attitude. They are not responsible for you having the nouse to install a GTK Theme application like lxappearance and fixing how your Firefox looks.

You can make things look pretty decent by running some kind of a settings daemon so that apps that follow xsettings Do The Right Thing. Either run one from one of the DEs, or find a thing called xsettingsd that's more standalone but a bit more painful to use. But I actually like it better since it puts the settings in a text file, and it doesn't also hijack shortcuts and the like (like gnome-settings-manager does).

I guess you are referring to the GTK/QT/... theme used? Most tiling WMs don't set one by default, but you can easily do so by using a custom .gtkrc-2.0 or so[1]. [1] see, for example, https://wiki.archlinux.org/index.php/GTK%2B#Themes

This is exactly right. GNOME, KDE and other out-of-the-box solutions make sure to set the GTK and Qt themes so that everything looks pretty. When you use a WM (window manager) rather than a DE (desktop environment), it is up to you to setup all the ancillary things such as themes and daemons. This is why .xprofile exists!

Also, I find lxappearance an easy and quick way to set GTK themes via GUI rather than having to edit the .gtkrc files for every option.

Yep. I use Ubuntu and my .xsession starts unity-settings-daemon before firing up Qtile to give me a more pleasent desktop experience.