I was hoping for a few very obvious and achievable improvements from the S4:
1) Industrial hardware design. Some indication that they have hired designers with taste. Premium feeling and looking materials. On this count they fell on their noses.
2) Some level of colour calibration. iPhone has it, Nexus7 has it, now the Z2 has it and it looks like they've put a huge amount of work into getting an exceptional gamut, but the S5 is presumably still going to be plagued by off-white, badly skewed 'vivid' colours.
3) A rethinking of Touchwiz. It looks like they actually began to do this in the Settings; and the promo material was strongly indicative of it; but then they did a bait-and-switch, and it turned out to be the same old bog-ugly UI design. The homescreen is identical.