Everyone write posts here, so I will make one too.

Here are my thoughts of this years themes.

Garden - a good idea for a theme, but for me it's too general and I would have difficulty making a game.

Decay - there are different kinds of decay, so this theme would be good, but there are some better themes.

Turn a violent genre non-violent - I don't think this to be a good theme, because most of the games can be unplayable or not enjoyable, because of gameplay not matching the genre.

An ancient language - it can slightly limit participants' creativity, but anyway I like the theme.

Butterfly effect - I'm a big fan of a butterfly effect but it can be hard to make a good game on this theme in such a short time.

Déjà vu - another time related theme. I like the idea, but again I know it may be difficult to make a good game in 2 or 3 days.

1 minute to prepare - I don't really like "you have to hurry" games.

Fragile - a good theme that traces a way to follow, while giving you a lot of freedom.

You are the dungeon - I once played a board game about being boss monster, and building dungeons and setting traps for heroes. It was good but I don't need to play more clones of this game

Grow a tree - this can be a good theme, but I can't really imagine so many games of tree-growing.

Side effects - life is all about making decisions. A game making you make decisions with side effects is like life.

Four elements - i like everything related to elements, so YES

Your game has 3 rules, but only one is active at a time - I love it! Games in which rules change while playing are a great mental exercise.

You are the princess waiting to be rescued - game about doing nothing and an waiting for help? No.

Equilibrium - there are many things which can be in equilibrium, so I'm looking forward to seeing a variety of games.

Dreams - there is a game named Dreams and it's the only game I want to see on this theme.

Momentum - physics-based game can by great, but for now I don't have any idea what I would do with it.

Sabotage - sabotage is a great concept in multiplayer games. Since most gamejam games are singleplayer, I don't know if it is the best.

Echo - that's an interesting theme to make a game, but unfortunately there is as possibility that most of them will be very similar.

You are the power source - I don't really see a game about being a spaceships engine.

Tower offense - while "tower deffence" is a game genre, "tower offense" can produce a lot of different games.

Mythology - that's a theme that influences setting, but doesn't affect the gameplay.

Fragmentation - idea of fragmentation sounds interesting, but will the games be interesting?

Turn a job into a game - if you make a game of a job, then the workers of this job will immediately get bored.

Evolve - I like this theme, but there are so many games about evolution that I got tired of them.

Replication - again, good idea, but personally I wouldn't know what to create on this theme.

You can't save them all - to have to make decisions is good in a game, but the fact that you can't save them all is too sad for me.

Control two things at once - super! Challanges of achieving something while changing many things are very popular in games because they make you use your head, understand and plan some things.

Stuck in the loop - this theme gives you a lot of different interpretations so it's very good for LD.

Déjà vu - is this a deja vu?

Multiple perspectives - I like the idea of the theme, but I don't like the choice of words (I'd like more poetical/less literal sound of the theme).

Déjà vu - AGAIN!? If people voted randomly it would have the highest probability of winning.

Outdated technology - old technologies, steampunk, ancient batteries! Super!

Magnetism - yes. I've never seen a game about magnetism, and there can be quite big variety of games.

Malfunction - good theme. Something not working gives you an objective in a game.

Ruins - game in this theme can be enjoyable to make, but it limits the setting of the game.

Connections - very wide theme, but not too general. There can be all kinds of connections.

Automation - factorio is a great game about automating processes. I would love to play other peoples ideas of automting things, but I don't know if it won't become boring after playing 50 of similar games.

Keep it alive - this theme gives you a lot of different perspectives and ways to make a game.

Reflection - the first thing that comes to my mind is light reflections, but I think that there can be more.

Planets - although I like this idea, unfortunately it limits the setting of a game too much.

Seasons - another theme giving you some idea to make a game, while not limiting the possibilities.

An abandoned world - this theme slightly limits game setting so it's not the best.

Protect the king - this theme sounds quite not bad. Although it may limit our creativity it more or less specifies the goal.

Shelter - since I started participating in Ludum Dare this theme has been in the finals. I see it again, but I still don't know if it's so good.

Inconvenient superpowers - similar to side effects (positive+negative effect), but not only (there can be superpowers that have only negative effects). Anyway, I like "side effects" more.

Delayed effects - the theme implies that you have to plan your moves ahead, so I like it very much.