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Something weird I noticed -- pressing and holding the up and left arrow keys, then pressing the space bar makes it so isKeyDown returns false for the space bar. Holding the space bar and then the arrow keys works fine, though. Why is this?

I could be wrong but isnt it normal that the OS only takes 2 key inputs in general? Ive tried what you said and I always get only 2 outputs. Always the 2 keys I pressed first. What I press after them is ignored unless I release one key.

Actually you should be able to press more than two keys... Can you show some code? (Maybe clearing the key input helps )

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It often depends on your hardware. For example my desktop PC can take 3 keys without any problem.
My laptop can mostly only take two keys but some 3 key combinations do work.

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Right Angle Games | Marte Engine
Back to the past | Star Cleaner | SpiderTrap

Okay that's new...
I made some tests, and I can push 6 keys simultaneously o_O (and more...)

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Current Projects:
Mr. Hat I
Vegan Vs. Zombies
Projects:
RadicalFish Engine - Build on top of Slick2D, Ideas, Bugs? Open an Issue ticket!

At least on windows you have that issue. Using two Arrow-Keys and Space-Bar at the same time leads to "beep". With a USB Keyboard you shouldn't have that issue, i know it from the good old PS2-Connector.

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Embero Games | Astroiz | ConK | From Outa Space

Pushing a different pair of arrow keys still lets the space bar work. This only happens with up and left.

Also, no beep here.

The issue might be your keyboard. With ~105 keys on a keyboard you would need >105 pins on the keyboard controller IC to read all pins at the same time. So most keyboards use a matrix to read the pins which will lead to restrictions on which and how many keys can be pressed at the same time. There is no norm or standard for this. But it is likely that you can use one horizontal and one vertical arrow together with the modifier keys (control, shift, alt).

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TWL - The Themable Widget Library

Oh how I hate this problem. It's entirely a keyboard hardware thing.
http://www.sjbaker.org/wiki/index.php?title=Keyboards_Are_Evil

There are a few keys that are guaranteed not to mask: Ctr Alt Delete Esc
And few that are very unlikely to mask: Shift Tab Enter Space Backspace

On old gaming keyboards it is not uncommon for the every key on the numpad to be isolated.
On new gaming keyboards it is not uncommon for WASD and the Function keys to be isolated.

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This is hardware thing but game dev can't blame user hardware so we should have figure best solution to this. Which are best movement and action keys for average player?
Movement is little easier. You just can do movement so intuitive that player never have to push more than two arrow key at same time.
But how about action keys? Is control good? Do space have more problems? Z and X are almoust standard keys if you have to pick a char.

Let's figure out best practise for this.

I think the link that Chronocide provided (that one: http://www.sjbaker.org/wiki/index.php?title=Keyboards_Are_Evil) already contains a good mapping of interfering keys.

Best workaround is to provide an option to define your own keyboard mapping for the game.

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Right Angle Games | Marte Engine
Back to the past | Star Cleaner | SpiderTrap

I just don't see that average player would try hassle with keyboard mapping. It's good solution if you just want other programmers test your game put it's not solution for final users.

Most / Many games have the option to change the keyboard / controller mapping. It's always a good move to allow people to adapt the controls to their style.

For example WASD is the standard-movement binding for FPS type games but i've seen other players which used ESDF or good ol' Arrow-Keys too.

Just my 2 cents

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Embero Games | Astroiz | ConK | From Outa Space

Yeah. Its good to have but same time we should make games where noone would need have to use it.

Basic usability principle: It's good to have manual but it's even better if user don't need to have it.