SDL is pretty barebones, if you want to build a UI with it. You've got to build up all your widgets yourself from scratch - rendering & compositing, handling input events, etc.
SDL really shines in simple cross-platform abstractions - window creation, basic rendering, input handling, threading, timers, basic audio, platform-agnostic file handling, etc. The older version (1.2) also had extensions for fonts, networking, and some other things, but I'm not sure if that applies for 2.0. Beats the snot out of messing with Win32 for getting started with game programming.
SDL 2 still has the _gfx, _image, _mixer, _net and _ttf extensions available. The only reason to still go with SDL 1.2 for anything new would be if you still want to target old, obscure platforms.
SDL really shines in simple cross-platform abstractions - window creation, basic rendering, input handling, threading, timers, basic audio, platform-agnostic file handling, etc. The older version (1.2) also had extensions for fonts, networking, and some other things, but I'm not sure if that applies for 2.0. Beats the snot out of messing with Win32 for getting started with game programming.