Mike, let me preface this by acknowledging Hopkins is a great actor and has great presence, however I agree about his casting as Odin, and most of it has to do with his physical attributes. A lot of it can be attributed to how he was portrayed in the film as being tired and such, and that Branagh wanted him shown in such a light. Hopkins voice seemed weak and he doesn't have an intimidating Viking presence but I think it will only be noticed by long time fans of the classic Thor.
While I don't think he was the perfect fit for the character, he did a great job, and there was certainly a much greater chance of finding a far worse choice. The only other issue I found with the casting was that of Sif. She is a goddess and the actress comes across as a cute field hockey player. Now I understand that you need someone that can stand next to Portman and not eclipse her beauty, but I think they could have had her more 'noble'.
My only other complaint is that the timeline seemed a little rushed. They could have had 'months' pass on screen instead of days.
Keep in mind that I think all these are very minor critiques and overall this was one of the most challenging Marvel comics to translate to screen and they did an amazing job. The most important issue is that this was Thor and he came off perfectly. Hemsworth did the work and got big enough to be believable. My favorite part was that Thor was portrayed as so incredibly powerful, the battle in Yodunheim, when he had enough and unloaded and single handedly whooped the frost giants, he dispatched the Destroyer, which inaccurate, to the comics but impressive and most importantly when it became clear he was holding back with Loki, he just leveled him. It will be interesting to see how they try and match the Hulk up to that power level, which of course shouldn't be done.
If they can keep this trend going, I don't see 'comic book' movies sliding back into obscurity as they always have in the past.
Portman seemed to walk through her lines, and for an actress of such renown I expected her to really stand out.