Nobilis seemed to have an awesome premise, and was intriguing due to being diceless. But each time I try to read it, I give up very soon because it's not easily approachable. So I am bouncing off this one even before playing.
Universalis. A game system meant for a lot of flexibility and universality, with people 'voting' with their coins for what each one of them considers important in a scene or worldbuilding. In practice, the worldbuilding phase felt like everyone was incentivised to add a gotcha twist to a previous setting fact by flying just narrowly around it and adding something subversive. Another player made a comment along the lines of 'the ruleset of Universalis is getting in the way of the playstyle of Universalis'. The bidding mechanics for resolution of disputes (IC and meta) seemed like a cool idea, but felt clunky in practice and mathematically suspect (though by now I forgot the details).
LANCER. This only applies to the setting, not the mechanics (I love the mechanics). In a medium where everyone publishes yet another grimdark / dystopia / post-apoc, I was cheerful to find a game that brands itself optimistic. However, upon closer examination, it became hard for me to take all the eutopian claims at face value, as the description of the Union (the designated big good faction) came out as either self-contradictory/inconsistent (branded as post-scarcity and with reduced private ownership, and yet megacorps seem to play a huge role in the Union, and there's a lot of artificial scarcity through licensing), hegemonic, expansionist, and self-righteous. This makes it hard to take the proclaimed eutopianism and goodness as written, which clashes strongly with the authorial stance that none of the descriptions are propaganda/satire/overhyped.
Mindjammer. A vaguely posthumanistic setting in the vein of Culture for FATE? Count me i . . . wait a minute. The alleged big good is memetically genocidal towards any creeds except government-approved ones to a dogmatic level, the techology seems to be spread in an oddly top-down manner, and the former two points combine to undermine posthuman stuff in an oddly WH40k-reminiscent manner. And there's a designated villainous faction that looks like a parody of WH40k. And then the system, which seems to have a stovepipe design antipattern: lots of minor subsystems for all sorts of activities that are based on overarching principles, each repeating many of the same words, but often with small differences, so it's difficult to memorise and inconvenient to look up, and hard to consider when deciding what kind of character you want.
Continuum/Narcissist/Seedless Bloom/Splintered Rose. Cool ideas about time travel. Extremely unhelpful manner of explaining the timeline cosmology/physics (a lot of the basics only become clear only after you read advanced supplemental material).