As for how humanoid an alien should be, I feel like it's all a matter of what you can manage to do with it. There are plenty of examples of aliens being very humanoid-looking that work out rather well.
First thing that comes to mind is Doctor Who. Even beyond the obvious example of the Time Lords, there are plenty of other alien races in the series (especially in the Classic era) that look exactly like humans, with no real justification as to why. And, you know what? It suits the show really well in most cases, because in spending less time trying to come up with weird alien qualities to make their extraterrestrial races less human, they can spend a lot more time developing the world and society that those aliens live in, often to very interesting results, since, being aliens, they can do virtually anything. And that is what makes the races alien, even if they look just like humans. Honestly, Gallifreyan society is one of the most fascinating aspects of the show to me -- one that I keep wishing they'd spend a bit more time on.
And then, meanwhile, you can have aliens that are really, really alien and that are really interesting to read about as they display so much creativity, but sometimes just... too much creativity. Animorphs is a prime example. I remember I loved all the aliens in the series because they were just so detailed and well-thought-out, so alien. But, after a while, their alien-ness started to become quite a flaw. I remember the later books in the series being a lot harder to get through because there were just so many aliens that were all so alien, they were honestly just really hard to visualize and wrap my mind around a lot of the time. It grew to be a bit much -- especially when they were so alien that there grew to be very little to relate to.
Still, Animorph's aliens were really nice in smaller doses, and when there were still enough human characters and human concepts to make sense of the whole thing. Detach all aspects of humanity and familiarity and you've got some difficult aliens for the readers to understand.