Going beyond the technical concerns, the thing that tends to make a game streamable is if either it offers the chance for exceptional, visually interesting play (take something like Ikaruga where it's just a pleasure to watch someone play) or if the game is so fatalistic, and has so many outcomes, that no two games are exactly the same and a game comes with many surprises. This latter reason is one of the main reasons that Battle Royale games have become such a big part of streaming.
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Good point. It now feels like some games are made almost entirely for streamers- like Goat Simulator. A lot of other modern indie games seem to be trying too hard to be weird for the sake of getting attention (but, then again, pretty much the only streamer I watch is Vinny Vinesauce, so I do see my share of crap games).
Since I only tend to work on graphic novel games, it's much more of a niche market (in terms of writing them, it's sometimes a balancing act between having a coherent story and an open world), but there are still variety streamers out there willing to give them a go...