The United States is still going to recognize Jerusalem as Israel's capital, so what impact does today's (12/21/17) vote really have?
No direct effect - the resolutions of the General Assembly are non-binding except on operations of budgetary/procedural work of UN itself (which this resolution didn't cover).
There may be minor secondary effects, mostly the resolution being cited as "evidence" by opponents of the move; or as evidence of UN's anti-Israel bias by opponents of UN, especially in US. It's hard to see how that can be considered "real" effect; but it may shape public opinions which in turn may have some effects on states' governments' policies.
I wrote an answer to a similar question.
First of, note that recognition of capital cities is meaningless. Russia has already "recognized" Jerusalem as the capital of Israel and no one bat an eye. What the US is doing, beyond that, is recognizing the Israeli occupied East Jerusalem as Israeli territory. From the Embassy act:
In 1992, the United States Senate and House of Representatives unanimously adopted Senate Concurrent Resolution 113 of the One Hundred Second Congress to commemorate the 25th anniversary of the reunification of Jerusalem, and reaffirming congressional sentiment that Jerusalem must remain an undivided city.
It is this recognition (not the one about Jerusalem being the capital of Israel) that the UN General Assembly (GA) is now condemning.
The GA has condemned Israel dozens of times for its policy regardin the occupied territories, but this is the first time, to my knowledge, that the US has been condemned for it. Certainly, it is the first time that the GA has condemned the Trump administration.
One can only speculate on what effects it might have. Previously, the US has played a role as a mediator between the State of Israel and the Palestinians, like Bill Clinton did at Camp David. But it appears incredibly unlikely that Donald Trump could ever play a similar role. His support for Israeli sovereignty over East Jerusalem quite clearly disqualifies him from that role.
Trump has also threatened to cut of aid to countries that voted in favor of the resolution:
"All of these nations that take our money and then they vote against us at the Security Council or they vote against us, potentially, at the Assembly, they take hundreds of millions of dollars and even billions of dollars and then they vote against us."
However, it seem far-fetched that the US would cut off aid to important allies in the region like Egypt. The Egyptian military regime is held under the arms by American aid. If that aid dries up, the Muslim Brotherhood could very well seize power. But you never know because the current US president is very unpredictable. The Trump administration and the rest of the world appears to be on a collision course.