*It's not a bug, it's a feature (-1) *- How is this a theme? This has long stopped being original or funny as a comment, but as a game theme? The only outcome I can imagine, is that every bug (and there are a lot of bugs in jamgames by nature) get's blamed on the the theme in a forced trying to be funny way, so: No, thanks.
*You can't save them all (-1) *- can be a solid gameplay feature, but having hundreds of games based upon this concept? I wouldn't wanna play a dozen of those.
*You are the final boss (+1) *- technically a bit restrictive as in "there has to be a boss", but if interpreted loosely enough, it's easy to stay true to this themes in a bunch of ways besides the obvious. I see especially strong narrative options with these, as in overcoming lethargy, self doubt, etc.
*Death is useful (-1) *- a good perspective for a specific type of game, btu not a good theme overall.
*Find it, protect it (-1) *- to sepecific.
*Shadows (+1) *- probably a bit to vague, but I like it =)
*Light is dangerous (0) * - a strictly worse version of the many light/darkness based themes already in the race, including "shadows".
*The game rules change after each failure (-1) *- nice gimick, but a) not a theme, b)if to be pulled off well not very suited for jamgames and c) too specific.
*Floating islands (+1) *- not original, but I like it.
*the more you use an action the worse it will perform (-1) *- too specific, too restrictive. This can be a good ability-management/balancing feature, but for many games this wouldn't apply very well.
*You control the environment, not player (-1) *- too restrictive.
*Craft your own magic (+1) *- a magic-based theme I actually like. Good job.
*I have been here before (-1) *- ominous sounding stuff can make good themes. But this is just too dull.
*You are not the main character (0) *- why do so many people want to do this theme all the time? Or is just one person suggesting it over and over? It's not really bad (a bit restrictive in regards to gametypes), but not really inspiring either.
*Shelter (+1) *- Solid suggestion. Mechanical as well as narrative options to use this. Many possibilities to build upon it, yet few restrictions. So far one of my favorites.
*Night is coming (0) *- hm. Sounds like a GoT-rip-off. Quite a few possibilities, but meh.

So I guess that is it! My favorit themes from rounds 1-3 are: * Shadows (Round 3) * Shelter (Round3) * Decay (Round 2) * Fragil (Round 1)

I guess I like flavorful, yet unspecific one word themes, huh? I wouldn't have thought that of myself but it kinda looks like it =)

Runner ups for me were: * Dying planet/Ancient world * Two points of view * Craft your own magic * Alchemy (& Magic)

Looking forward to the final round =]