Its hard and easy to tell at the same time: the less the better, the more the better.

everything depends on your needs. and detail should only be put where it makes a difference and where it can be seen.

Example:
+usually the shoes of player characters are less detailed then their heads. harldy anybody looks at them anyway.

+ceiling textures are about 50 or 25% of the size of the wall textures in fps shoters. you dont often look at them, so you dont need lots of details.

+houses in fps games have usually different level of quality depending on their height. 1floor = full details, second floor = only outlines and structures are made (frames for windows, parts of the wall aso), 3rd and above most things are texture painted or just basic shapes.
The further away your view is the more waste would it be to use polygons and texture space for it.

Highest floors in hl2 buildings (outside walls of it, 5th, 6th aso floor) also used a second texture space on the uv-sheet.
they basicly placed the same texture just in very low rez on the same texure sheet and used it for floor higher above the ground. safed them lots of power

+interiors for cars and vehicles only if you can enter them


the reason why developer games look usually a lot better then indy games is simple: its their dayjob
if every indy could work 10h a day on his stuff he can create the same things.
If you can collect enough indies you can even get the ammount of content a developer game has.


one thing i add to every of my starting levels: "the chunck"
The chunck is a 15.000 polygon, animated "thing" that is placed into every view.
This usualy leaves enought space for enemies or code. I also place enemy placeholder into the level where they will alter on act.

one feature i would love to have is a polycounter in WED.
not only for the whole scene but arround a defined sphere from the middle of my mouse pointer.
Move your mouse onto a point, press a button and every face arround it will be counted and diplayed in WED.

cheers