All of the different major companies in gaming have their strengths and weaknesses, and one strength that's definitely in Microsoft's corner is their ability to associate and negotiate it with outside companies: in the case of gaming, that means third party publishers. I think their strength in this area is virtually unparralleled.
There is no concrete proof to verify this, but there is so much anecdotal and partial evidence: the GTA IV situation, where they managed to not only secure a day-and-date simultaneous release, but also to secure exclusive content; the Guitar Hero and Madden ads, which prominently displayed the Xbox360 logo and managed to help secure the Xbox360 as THE Madden-system of choice for the generation; the system bundles with third party software such as Marvel: Ultimate Alliance, and even this recent public event in Japan:
Where Microsoft clearly teamed up with Namco to provide not only Viva Pinata mascots (which are first party) but Katamary mascots, as well (which are decidedly third party). The number of promotions and financial deals and marketing collaborations between Microsoft and third party publishers is unparalleled, and it has clearly had a beneficial effect on the system, which has seen extraordinary third party support.
Any comments or criticism? Just as Nintendo has thrived off its strength in innovation, I think Microsoft's surge in popularity and support is a direct result of their extreme hard work in getting third parties aboard.