I sort of agree with you. There are some changes while I didn't like, I at least understood.
From what I know about FF#48 (the original Galactus story line) both the Watcher and the Ultimate Nullifier are really more plot elements then real characters. Remember, what works in one media doesn't always work in another. An all seeing alien showing up to tell the Fanataic Four about a dues-ex-machina all powerful weapon might not have been doable in the movie. (Yes, I know they've done it in the cartoons). In the years since, both the Watcher and the Nulifier have been explored, (although the wiki page about it still seems somewhat vague to me) but to they probably wanted the action to focus on the Fanatic Four and the Surfur (who was supposed to get his own spin-off movie)
Another change they made, is one that is propably a misconception a lot of people have. The movie states that the Surfer's power comes from his board. This is incorrect. When I saw it in the movie, I went "WRONG!" The Surfer is bestowed with the power Cosmic from Galactus and the board is an extension of the Surfur's power-not the other way around. He's able to form one at will. There's a mini-series where the Surfer states the reason he creates a board is so his view of the universe isn't hampered as he travels through it.
As for Galactus. Some comics have stated that Galactus' appearance actually differs from planet to planet. Earthings may see him as a large humanoid, while another might see him as a large lizard, or a squid-like god of destruction (as in the Beat Ray Bill mini series). They also talk about him as being more of a force of nature-which is what I think they were going for...but I still didn't like seeing him turned into a cotton candy cloud with a molten carmel core. Heck, I might have actually preferred the fleet of living ship that he is in the Ultimate Marvel Universe. At least that would have been a more concrete concept.
There had to have been some sort of middle ground between the classic Galctus and the more alien eneity they wanted to portray him as.