Like yourself, I have been using a D300 and a D300S for the last three years and have enjoyed immensely the results I was getting, and again, like yourself, having only then gone from film to a DSLR and having achieved decent results there, I thought it was time to step up the game a tad lolol
Enter a Nikon D800 and a very nervous photographer!!! As all have said here before me, the additional resolution is not anything like as forgiving as the D300 and this is highlighted even further as one moves up the focal length scale. I have found that by going back to basic techniques of shooting, like taking a deep breath at shot time, tucking in the elbows etc has helped me hugely. I am not for one minute suggesting you don't have these techniques, but I am sort of saying that for me, the D300 allowed me to go backwards into bad shooting habits because of its forgiving nature, this sadly is not the case with the D800.
I have no doubt you will find the D800 a fantastic camera and whilst it might take a little more initial work, it wont take long to fine tune and get astounding pictures again.
Good luck and please let us know how you get on.
BTW I agree wholeheartedly with the comments posted about criticism, that to me was the beauty of joining Nikonians in the first place, nothing was too simple to ask and everyone (or almost everyone) has always been extremely helpful and long may that be the case.