Most things listed have labelled screenshots.

E1M1 - Some door textures do not look like doors, a nice framing of the doorway to be more door-like seems easy and more visually obviously.
E1M1 - Theres nonblending texture changes on a flat wall, could be slightly sunk with an extra sector or a few linedefs to frame spot a little better. 
E1M1 - Feels a bit clippy on jagged cliffs, couldnt actually go out of bounds though. This was an issue at some point, probably fixed.
E1M1 - Exit switch seems higher to match wall texture, but then frame texture is doubled on bottom.

E1M3 - First door has no frame, just tracks. 
E1M3 - Many areas throughout map have the startan texture a bit off. Looks like it was auto aligned. Maybe adjust some geometery slightly and manually align textures for cleaner edges and corners. Can also add borders to edges and corners, to help for perfect alignment. Maybe instead of borders or geometry adjustment, add a vertex to visually stretch or shrink texture, line up edge of texture with edges or corners. 

E1M4 - Also many startan alignments.
E1M4 - Room right outside exit can probably have cieling lowered 8 pixels to align entire room's textures. 

E1M5 - Lots of ammo. Seems monster floods removed or teleports broken? (some reason ending seems much easier).

E1M6 - More startan alignments. Screenshot shows a stretched startan, but for no obvious reason making it align worse with geometry. 
E1M6 - Switch for small door to outside do not seem linked. Switch area could be expanded into outside area a bit and switch moved. When player hits switch, they are facing same direction and have a clearer view of door. 

E1M7 - Some floating items.
E1M7 - Some odd flats.
E1M7 - A switch looks odd, maybe just a normal door is fine. Not much space for a switch in that room. 

E1M8 - Rock maze is a rock maze. Fight is very easy, should have more much action in both areas. Very short and boss map can ramp up skill.

E1M9 - Minor texture alignments.
E1M9 - Peg doortrack.
E1M9 - Door texture used as support.