I wouldn't worry about character sheets, for the most part. Blank character sheets are essentially a form, something that isn't protected material. Anyone can pretty much make a form for anything, including for use with other peoples material, systems, mechanics, or products. However, it can't contain copyrighted IP on it. If your blank character sheet has pre-printed IP on it: such as feat descriptions; skill descriptions; powers, and lists of these items (except skills - I don't know of any skill that has a name that isn't a common use word) - in other words, verbatim text from WoTC products, then they could be targeted. If they also have drop-downs containing that information, they may be targeted. But, a basically blank character sheet, even one that has labels for things like: abilities, skills, hp's, etc. - should be okay since it's basically a blank form. However, filled out character sheets may also be open to targeting, but I don't know about this one. Maybe some of ENWorlds resident lawyer members could weigh in (paging
joethelawyer).
Along the same lines, I would think (as some others have already said here) that blank power cards, and power cards with original fan-created material (not verbatim WoTC IP text) should be okay. You probably don't even need to sign the GSL, as long as you don't specifically mention Dungeons & Dragons, WoTC, Fourth Edition, etc., or use any copyrighted material or terms. Of course, everyone will know what they are for, but as long as you don't specifically say it, I
think you'd be okay. Now if you start charging money for them, all bets may be off, but I'm not sure about that.
Of course though, as others have said before, even if you are legal, a cease and desist could be issued anyways. In which case it's up to you to fight it or not. If someone doesn't have the money to fight, even if they aren't doing something illegal, and shuts down because of it, I guess it doesn't really matter whether they were right or not. However, in all fairness I've never heard of WoTC doing this. (TSR did it as a matter of course, and it worked, except for the whole destroying the industry thing

- but I've never heard of WoTC doing it.)