There are minority-shareholder issues which should not be underestimated. However, there are plenty of other issues as well.
Let's imagine you have a board of directors with 3 members. Our pretend company is still privately held and all the board members are shareholders and all shareholders are represented on the board. Let's even make it more interesting saying that the percentages of ownership are as follows:
Board Member A: 51%
Board Member B: 30%
Board Member C: 19%
At this point, Board Member A is certain to win any vote since he can cast all of his votes and override anyone else. If the company goes public, that is, the company sells shares, this could (depending on the structure of things) dilute Board Member A to the point he has less than 51%. At this point, the votes could go any way.
Another thing to consider is that bringing in new owners (e.g., by a public sale of stock) allows those new owners a voice - and a vote - in the operations. A may feel comfortable with B and C but D (a new member) can also attempt to influence B and C. If D is successful, A could find himself in trouble.