Primarily the manner in which they contemplated the purchase of Nintendo. It was not presented as wishful thinking, but an inevitability. Microsoft has the money, and all they need is a moment of weakness on Nintendo's part in which to spring a hostile takeover (or at least, that's how the email reads).

The fact that Microsoft could leverage its considerable wealth from being the market leader in a different market segment to buy out an entire competitor that makes up a very significant portion of the gaming market, and that doing so would not be for any reason than to be a feather in the cap of the Phil Spencer makes them very dangerous.

Xbox/ Microsoft clearly have money to burn and are on no uncertain terms willing to use it to bully their way into a monopoly of the gaming market.

Regardless of how realistic their plans to take over Nintendo really are. They think the plan is feasible, and they have the money for it. They are incredibly dangerous.