I bought Wolf3D for the iPhone on a whim, expecting it to be your typical iPhone game with stuttering gameplay and sluggish controls, but it really surprised me in how well it played.
I think it's easy to just think of Carmack as a genius programmer but he also really knows how to tune a game to be fun too, from reading Carmack's blog about developing iPhone Wolf3D it's clear he put a lot of thought and effort into streamlining the gameplay. He removed the high score from the game as it was fairly superfluous. He also dropped having a finite number of lives in the game so you can restart a level whenever you want. He rightly noticed that too many controls are a pain so he made door trigger automatic (like Quake). Best of all he added an auto-map to help make sense of the maze-like levels (using the old maze solving algorithm of hugging the left wall works well for most levels ;) ).
On top of all that he also wrote a game that maintains a steady frame rate and has very responsive and comfortable controls (which is very unusual in iPhone games). So, with all that in mind, I'm really champing at the bit to get his port of Doom Classic, but it still hasn't been released. I can only assume they're waiting for Doom Resurrection sales to drop off before releasing another Doom game to avoid cannibalising sales. Doom Resurrection is OK, but the tilt controls aren't that great and being on-rails it's missing the fun of being able to explore a level yourself at your own pace.
Apparently Duke Nukem is on the way as well. I hope they do as good a job of porting that as Carmack did with Wolf3D. Now if LucasArts could be persuaded to port Dark Forces and Jedi Knight.... ;)