Q: I think Christopher Paolini used ideas from [insert book/film/video game here] in the Inheritance Cycle - can we mention this in the article?
A: Only if this similarity has been mentioned in one or more reliable sources. Blogs, forums and fan/hate-sites (such as now-defunct Anti-Shurtugal) do not count as reliable sources. If the similarity has not been discussed in reliable sources it cannot be put in the article, as this violates Wikipedia's policies of Verifiability and No original research.
Some similarities have already been mentioned in the article. Similarities to Star Wars, Lord of the Rings and Dune have already been mentioned in the article, and the corresponding reliable sources are given. However, the similarities aren't explained in detail because the sources cited don't explain the similarities in detail.
Q: Can we start a "Criticisms of the Inheritance Cycle" article?
A: No. Such articles should only be created if there is not enough space to cover it in the original article (for example, ESRB re-rating of The Elder Scrolls IV: Oblivion was created because the original section at The Elder Scrolls IV: Oblivion#Rating change was getting too big). The Criticism section of this article as it stands is quite short and will need drastic expansion before an article by itself can be considered a benefit. If you want to add details regarding the series' reception (both positive and negative), feel free to do so in the already-existing Reception sections on the relevant articles. If you do, make sure you provide the corresponding reliable sources.
In fact, a couple of years ago there used to be a "Criticisms of the Inheritance Cycle" article. However, it was deleted (see its AFD entry) because there wasn't enough content to make the page long enough. In order to make the page long enough, people added original research and tried to list every single criticism they could find.
Q: Should we add spoiler tags before the Character and Story sections?
A: This is covered by WP:SPOILER. You're going to expect spoilers in a section describing characters or the plot synopsis - it's common sense. Spoiler warnings are therefore redundant.
