To add to all of this, most people really don't understand the tax implications of options (whether they're ISOs, NSOs, or not options at all but instead are RSUs). It's hard to explain those without looking at a person's individual situation (the company, the initial value, the current valuation, and the person's tax bracket), which is why there's not a lot of talk about them. But the taxation can very easily be the…
I had options that I could exercise (at a big, publicly-traded company, so I didn't have the startup, no-liquidity issues to worry about). My options were worth quite a bit of money. I had already exercised enough to put me at the edge of the next incremental tax bracket. On about December 15, I was looking at it, and thought it was a really tempting price, but... taxes. I decided to wait for January.
Three days later, our company made an offer on another company. The stock market didn't like it. Our stock went down. I lost more than the taxes would have been.
Moral: When money is growing on trees, pick it.
Disclaimer: That isn't always good advice. If the price had continued to go up, it would have been really bad advice. But too much focus on taxes is also bad advice.