There are problems I have with the system that have nothing to do with marketing, but for me the marketing really did make me much more skeptical and critical of the product than I would have been otherwise.
Major mistakes:
1) Tried to tell rather than show: One of the biggest problems I had was with the marketing making extravagant claims about the system for which they were unable or unwilling to produce evidence. There were promises that they made about the system that I knew that they couldn't deliver on, features that they claimed existed that I knew wouldn't be in the final product. Because of that it made the whole product seem like nothing more than hot air, and made the real product suffer in comparison to this imaginary one they created with thier marketing.
2) Treated the fan base hostilely: The huge success of d20/3rd edition was treating the fan base respectfully. They solicited input. They did previews. They had a lengthy testing period. Fourth edition was rushed to market amidst a cloud of secrecy with problems that just one or two weeks of open play testing could have fixed. All I can say is that it is a very good thing they vetted at least some of the product, because otherwise we'd be dealing with 'Amber Dragon Tail Swipes' or some such for martial exploits.
3) Treating thier past products hostilely: This was probably the craziest one of all. Almost all of thier marketing seemed to emphasive that past editions of D&D were unfun, tedious, flawed pursuits that made you stupid and geeky to play, and how when 4e would come out that would all change. The problem of course is that if you did enjoy past editions, describing those editions in highly negative terms led at least me to believe that the new game was going to drop alot of the stuff that I did enjoy about past editions. Likewise, if you've been a market leader for going on 40 years now, don't fix what isn't broken. If you have a brand as valuable as D&D, why in the heck would you run it down?
4) Failure to realize what had made D&D great: Coming out with something first is no gaurantee that you are going to dominate the market. In fact, alot of the time, the second or third product in a particular market is the one that ends up dominating because it can learn from the successes and failures of the earlier products. So why did D&D manage to survive and thrive and lead the market despite mismanagement and healthy competition? It wasn't by accident. If I may insert a contriversial hypothesis, it is because D&D isn't a Forge game. D&D isn't gamist, or simulationist, or narrativist. It's a hodge podge of different rules and goals, some of which are seemingly contridictory, that leaves D&D without a really core gameplay. What this means is that D&D isn't a master of anything. For any game style you might want, there is probably a better designed system. But it also means that for any random group of 5 or 6 gamers, there isn't a system which gets in thier way less than D&D. It is a concensus game, not a designed game. What 4e does is take the mode of D&D's play styles, and explicitly designs for it. That's great if you happen to be playing D&D according to the common mode, or if you want to play D&D but always felt it didn't do its core gameplay well, but if you happen to be someone who has played D&D for some number of years in an unusual way - and based on my experience that is most of us - then it's a disaster. What 4e does in that case is take some aspect of D&D that you might not have liked, and in fact the aspect of D&D that you found most problimatic, and emphasised it at the expense of how you were playing the game.
4e does exactly what Mearls said a game should never do - tell you how you should be playing it rather than giving you the tools to play the game you want to play. And the marketing, rather than letting you decide for yourself, just beat you over the head with this fact. It's selling point that it was trying to hammer home was a negative as far as I was concerned, and all the marketing did for me was hammer home, "Don't give this game a chance. It's not worth it. It isn't designed for you."