What am I doing wrong?

[sir_hatless]

I've bought the game and it starts up just fine, but most of the time it gets to the opening screen and just stays there no matter what I do. I got it to start playing once, so I know it can run on my computer, but I don't actually know what I did. Does anyone know if this is user error or an issue with the game?

[Unknownno]

Get a sourceport: http://zandronum.com/

If you want it to run like it was made get this: http://www.chocolate-doom.org/wiki/index.php/Chocolate_Doom

[Lurpsukka]

Have you tried to change output=source to output=ddraw in the ultimatem.conf (or ultimate.conf if you launch the game with classic controls) file? It's in SteamApps/common/Ultimate Doom/base

Don't forget to correct the aspect ratio to 4:3 by changing aspect=true

[Strajder]

Try this guide:
http://steamcommunity.com/sharedfiles/filedetails/?id=256726153
Basically, Steam version has settings that are suboptimal in the best case, and can make the game unable to start in the worst case.