I was really wondering what the actual difference on the chessboard is. That is, how many moves of a 2000 would a GM improve on / find problems with?
Could it be that the difference betwen an expert and a GM on the board translates only in few moves, few positional inaccuracies that an expert does but a master doesn't and these few moves are the ones that decide the game in the end?
In other words, I don't think an expert would be outplayed every move...just like even a beginners does perfectly good moves like half the times.
What do you think, am I just wrong?
I'm +2000 Elo national (+1900 FIDE) and I play various games against +2100-2300 and I make points. They are better but I can Fight and draw or win some game. I play games against a GM +2500 of my zone and always I lost without any chance. In my opinion the diference betwen a expert and GM is really big.

Plutonia, I once read that a GM will make about 1 move in 20 that an expert wouldn't make. In other words, at least twice a game, a GM will outplay the expert.
But iFrancisco is correct. More than individual moves, the GMs have a better sense for where the pieces belong. When I watch the top players, I am constantly amazed by their plans. They play at a different level of complexity and find ways to keep the fight going that are completely beyond me. I can often guess the correct move in a live game, (I do better guessing Kramnik's moves than I do Aronian's moves), but the plans are what really separate the strong players.
I once had lunch with GM Lein and Pavel Blatny. Lein argued that in order to play at Carnegie, you should be trained by someone who has been there. He thought IM coaches such as Dvoretsky, while successful, were not good enough to create a world champion. Perhaps he was right. But Dvoretsky's students have been good enough to play as candidates for the world championship.
So, while there's room in the world for coaches who do not play particularly well, if you are looking for a coach to make you a champion, you may well want someone who's been there.