It is a broad question, and I wanted broad answers. 
And yes, I am just kind of getting a feel from the community what their tastes are...
...in relation to my AB3D FPS.
I've been following your work, and, needless to say, I am a great fan
Now, let me try to help as much as I can...
First, a couple of broad answers
Every game needs the "right amount of levels". Well, except those which have no levels :crazy: You can't make, like, four levels, and then leave the player disappointed. Nor can you make 256 levels, and make everyone die of boredom somewhere around level 173. First AB3D is the example of perfection in that regard. Just make a well rounded game, and then, if you have more ideas, add them with a mission pack, or as a slightly updated sequel.
Then, you need to have some memorable stuff ! It can be anything really. Like some immortal tunes we are still humming some 100 years later. But not everyone is talented and music making can be kinda tough. Still, it could be a really cool sound effect. Record yourself or a friend for a really scary/funny monster effect. Invent a really unique/original weapon. Make it ultra powerful and fun to use, but make it ultra scarce also. Add a dash of humor, put up an "alien toilet" sign inside of the alien fortress, or an Amiga inside a computer room. Put hidden messages throughout the game. Let them tell a side story. Funny or dramatic. Happy or sad. Put an extra effort in one of the bosses. Make it super cool. It is impossible for all bosses to be super cool, but that one just might be, so everyone will remember him. Invent names and artifacts. But don't just splurt names like Xaxxzorx. It's on the wrong side of silly and reveals a lack of imagination. Mix artistic elements. Is a game 3D or pixel art ? Have a talented friend draw some cool pictures with pen and paper. Scan them and show them between the levels. Mixing art types goes a long way in describing an imaginary world to your audience. Everyone will remember "that game with cool drawings". Add some voice acting to the game. We are not so limited with RAM today. Make some memorable quotes for your audience.
Now, for the FPS specific. DON'T make a "save anytime" feature. It completely destroys suspension and fear factor. Save should be possible only between levels. Or just use codes for that purpose. Once again, AB3D got it just right. DON'T ever never ever do enemy re-spawning. It's annoying as hell and unrealistic. It could be done only as a part of a story/level design. Like there's an alien teleport and they keep popping out until you shut it down/destroy it. But re-appearing enemies for no reason is a big no-no.
I'll be sure to ease players into the game play then in the first level or two, but by level three I'm going to turn up the heat...
...but with a little exploration, players will find items to help balance out the difficulty.
This is the right way to go. It should not be mandatory to explore the whole level, but if you find it too difficult, you can snoop around and find that one grenade that will ease up the fight a bit.
Level design. As harsh as the constraints might be, try to make the command room look like a command room. Cafeteria, locker room, ammo depot, shooting range, prison, engine room, medi-block... They should have some distinguishable details. Not just be a room with an explaining sign on it.
Whoaaa man, have I got myself yapping with no end. Well, I have much more to say, but I don't want to risk annoying people
If anyone needs me, I'll be in my bunk
