Well I'm late on getting around to this, and two rounds of voting are already open, so I'm going to group them both here. It's gonna be a long one, strap yourselves in.

ROUND 1:

Space Exploration (0) - A mediocre theme, not bad, but also not good. One I'm willing to work with, but I would rather not win. It's open, and I like that, but theme is something I like to have control over, and I don't feel like I would get that here.

Everything has a price (-1) - Have you heard of a little event called LD43? Spoiler alert, this theme is almost identical.

No upgrades, only downgrades (-1) - See above.

Color changes everything (-1) - Just too complicated for a weekend based off of my ideas for it. I'm sure it could work, but I'll pass for now.

Four elements (0) - Another theme that is just barely on the edge between -1 and +1. I would be glad to have it over some other themes, but it wouldn't be my first choice.

Mutation (+1) - This is what a good theme is: a single word, very open to interpretation where everyone can have a different idea. More themes like this please!

Floating worlds (-1) - Take three guesses why I'm not a fan.

Limited memory (0) - I feel like a lot of people did that with "Running out of space", and I'm not anxious to see those again, but I still would enjoy the theme.

Your life is currency (-1) - A very specific theme that, while it could have variation in genre, the central idea isn't open to much interpretation.

Symbiotic relationship (0) - A theme that could end either with every game identical or extremely unique. I hope it's the latter, but I can't take the risk and give this a +1.

Control the environment, not the player (-1) - Both A.) too complicated and B.) I'm just tired of seeing this over and over. It's almost at "floating islands" levels of hate for me.

Old technology (0) - A good theme, but also the theme for LD36. I could go either way on this one.

You have 30 seconds (-1) - I prefer strategy-type games, and not action, so a time constraint isn't exactly what I want.

Unusual magic system (0) - It's a good, interesting, open theme, but it requires magic, and not everyone might want to do that.

You can't kill the enemies (0) - It could be interesting, but I'm not sure I fully get it.

Parallel Worlds (+1) - It requires nothing except two worlds, and they don't even have to be that different. Yes please.

ROUND 2 (aka the bad one):

Cards are your weapons (-1) - Painfully specific. No.

The butterfly effect (+1) - One of the few themes I liked in this hellhole of a round. Open to interpretation, and one that could have a variety of outcomes.

Only one enemy (-1) - Implies combat, not everyone may want to make a combat-based game.

Light as a weapon (-1) - See above

One room (-1) - Heard of an event called LD37? 1 million dollars to whoever makes the connection.

Regular Jobs, but in Space (-1) - First, use proper capitalization. Second, it just writes the setting for me. No thank you.

The environment is a weapon (-1) - See "Only one enemy"

Change the genre of a classic video game (-1) - This is painfully specific, and would require a derivative versus an original work.

Decay (+1) - See "Mutation"

Stay in the dark (0) - A good theme, but a little specific for my tastes. Not the worst, but not great either.

Side effects (+1) - Side effects to what? What kind of side effects? Allows any genre, style, setting, mood, without being too vague.

Two-button game (-1) - Sucks for makers of complex games (i.e. anything that's not arcade-style).

Time is the currency (-1) - See "Your life is currency"

Risky powerups (-1) - This one isn't actually all that bad, but there's something about this theme that just speaks to me in the wrong way. I can't reason myself here, but I don't like it.

Permanent damage (-1) - I have a tendency to make games too hard anyway, so enforcing a difficulty isn't ideal. Also implies some form of combat/life meter/obstacles.

Don't let it escape (+1) - This could be taken in so many ways that I can't count all of my ideas for it on two hands. A broad theme that can fit whatever a developer wants to do.

So these themes were good. Until round 2. Then they weren't. I look forward to round 3, and all of the themes that don't suck (but they probably will). See everyone around in 4 days!