Reputation of the Federation
As with any organization of size and history, the United Federation of Planets has gained a reputation not only among its neighboring populations but those beyond. Some of that reputation is based on the dissemination of factual information, some of it is fabricated or exagerated for various purposes or simply through negligence.
Among Klingons
In the 23rd Century, the Klingons created propaganda claiming that the Federation had committed atrocities against its prisoners, keeping them in death camps along with torturing them for scientific and military secrets. This propaganda colored interactions between the Empire and the Federation, not only in 2268, but in 2290. In the 24th Century, many Klingons felt that the Federation was soft and weak, which led to the eventual Federation-Klingon War with some prodding by the Dominion infiltrators.
Among the Dominion
The Dominion's leaders considered themselves superior, yet the also viewed the Federation as a threat. Following the Battle of the Omarion Nebula, culminating the elimination of the Obsidian Order and the Tal Shiar, it was felt the only threat left within the Alpha Quadrant was the Klingons and the Federation which they would do what they could to pit both against each other. It was believed that the Federation would never go on the offensive against the Dominion, something in the end that would come back to haunt the Dominion when it lost to the Federation.
Among Others
Many desired association with the Federation. By 2360, multiple governments had requested membership into the United Federation of planets. Amongst these were the governments of Angosia III and one of two of the governments of Kesprytt III. In 2369, the Bajoran Provisional Government chose to become affiliated with the Federation, entrusting much of the operation of Deep Space 9 to Starfleet.
Some had a more nuanced view of the Federation. Quark, for example, considered the Federation to be like the Earth drink of rootbeer - "bubbly, cloying, and happy. And if you were around it enough, you began to like it, which made it insidious."
A rare few felt that the Federation was old, that it had been challenged by every major power in the quadrant - the Borg, the Cardassians, the Dominion - because they could smell the scent of death on the Federation.