The bottom line is board composition should be proportional to ownership. If a VC buys 25% of your company, they should have 1 of 4 board seats. The day investors (or their close friends who are identified as outsiders) have more than 50% of the board seats is the day you as a founder have lost control of your future.
I think most investors are fair about board composition up front. However some bigger VCs rely up on the fact that over time, you'll need to do multiple rounds, and they know that follow-on investors are frequently afraid to go against their decisions, so in effect, they end up with multiple board seats because the other investors don't want to piss them off. I have seen this dynamic firsthand, and it is not good. I hope this helps you!
I ask because I have heard many nasty stories of founders thinking that they would stay in control of their company, only to find that, because of the board, this wasn't true at all. I would like to know how to make sure this doesn't happen. It seems like the issues can't all be obvious, or so many smart people wouldn't fall prey to this pattern.