I already have the extended edition on DVDs. Other than HD, what extra do these Blu-rays offer?
Here is an Amazon review [amazon.com]
which addresses that very question.
Quoting the end:
If you own the extended DVD set, then buy this only if you want to have a great improvement in pictutre and sound quality. The only extra features you get is 'behind the scenes' DVDs. All other extra feature DVDs (total 6 of them) will be a duplication of what you already own in Extended DVD set. Actually, I noticed that all the extra feature DVDs here are from the various older DVD editions. The two DVDs are from the Extended DVD set, and the one 'behind the scene' is probably pulled from the limited editions of LOTR (the double sided DVDs that New Line Cinema released sometimes back)."