Out of the box, rxvt-unicode is not too pleasant to look at. Changes to and understanding of Xresources is required for updating its appearance. That being said, the experience of customizing it can be very rewarding.
Some people feel that Terminology has too many features that are not suited for a terminal but for a window manager instead. Things like viewing thumbnails, watching videos and gifs and other flashy things like that feel just like eye candy and should not be part of a terminal emulator.
Many terminal applications, such as OS X's Terminal.app and iTerm2 (among others), all claim xterm or xterm- variants as their $TERM and aim for support of xterm's escape sequences. Many command-line applications will assume or even hard-code escape-sequences and behavior for xterm and tho...
The default configuration may have some annoying behaviors (they can be removed by changing the default config though) like asking for confirmation when reusing an existing window or when starting the terminal to launch a specific command.