Here's one of the more famous anecdotes quoted here from George Washington: The Founding Father (Eminent Lives) by Paul Johnson:

Everyone else, in varying degrees, was astonished at this singular failure of the corruption of power to work. And, indeed, it was a rare moment in history. In London, George III questioned the American-born painter Benjamin West what Washington would do now he had won the war. "Oh," said West, "they say he will return to his farm." "If he does that," said the king, "he will be the greatest man in the world."

Another version replaces Benjamin West with an ambassador. I'd like to think it's true given George III's own fondness of the farmer and that he was kept abreast of American political developments.

I can't find a quote from King Muhammad III of Morocco but he had a very favorable opinion of the United States and presumably of Washington as well, being the first nation to recognize the US and also signed the longest unbroken treaty with the US in 1786.

This is also kind of cheating but the Maquis de Lafayette also wrote glowingly of Washington throughout his lifetime and would have praised Washington's Cincinnatus-like restraint especially as he participated in the French Revolution.

In part due to Washington's precedent, America did gain a reputation as a backwater refuge in the early 19th Century before heavy industrialization as a place to escape the Old World entanglement of nobility and social stratification.

This article from the Boston Globe's "Ideas" section doesn't address leaving power at all, or Europeans, but you may still find it interesting. It focuses on Washington's taking power, and specifically the debates about which honorific to use with the Presidency: "His Elective Highness, President Barack Obama" (the article is about Washington, not contemporary politics). Needless to say, the lack of pageantry associated with the presidency disappointed some people in America.

It's short and well written, but to give people a sense of it, here's a preview:

The Senate went on record to recommend the title of “His Highness the President of the United States of America, and Protector of their Liberties,” which senators saw as in keeping with global title conventions. As the debate unfolded in three weeks of closed-door wrangling, the chamber also considered some curious hybrids, like “Elective Highness” and “Elective Majesty”—whose strangeness to present-day ears is a testament to the novelty of the American experiment at the time.

I'll answer the title question first:

There's an apocryphal account that British King George III reacted with admiration and shock to the news that Washington would resign his commission. The King is said to have asked his painter, Benjamin West, what Washington would do after the war. When West replied that Washington's fondest desire was to return to his farm, the King replied:

If he does that, he will be the greatest man in the world.

The exchange is quoted as fact by many -- including most biographies of Washington, and other secondary sources on the Revolution. But I've yet to see a primary source for it. A thread on /r/todayilearned -- I know, I know, but bear with me -- actually took up this very question, quoted and identified several secondary sources, but found that the anecdote couldn't be traced to a primary source. As noted in the thread, though, the secondary sources are numerous. I would be very curious to hear where the account comes from.

Let's set that to one side, then: whether world leaders remarked on Washington's resignation, others certainly did. American leaders at the time certainly played up the significance of Washington's decision. The moment of his resignation is immortalized in a painting by John Trumbull. Washington entered Pennsylvania to perform that act under a wooden triumphal arch adorned with the likeness of Cincinnatus, the semi-legendary Roman dictator who Livy says saved the city, only to resign as dictator immediately once the danger was passed (Livy III:26-29). The new nation, in other words, took every step it could to make Washington's example a memorable one. The effort was not in vain: Byron's Ode to Napoleon Bounaparte makes the equivalence explicit in its closing lines.

To your other question, whether the resignation was a surprise, it was not to those who knew Washington, and Washington removed all doubts in his "circular to the state governments" of June 1783, which he closed by saying that he intended to "bid a last farewell to the cares of Office, and all the employments of public life." Someone else -- there are some excellent flaired users on Revolutionary history -- might be better suited to answering the question of what the nation's expectations were of Washington.

I have always wanted to ask this question, so I ask it now as a followup question:

Since the Revolution was mostly about secession from Britain, did anyone think about declaring their own King, instead of a Republic? Was the idea of Republic intrinsically connected to the idea of secession?

Side question- was there a similar reaction after the election of 1800? My ap us teacher always said it was important because power shifted from one faction to another peacefully. Did people expect the previous party to take control?