Well, it's more about if you know his deck, he knows yours too. So it's not about having THE surprise that your opponent had this whatever unit you weren't that much expecting.
To further illustrate the point, you must also admit that a faction doesn't have a single way to play. For example, the Guild Dwarves can go at Wall Destruction, at solo champions, at heavy lockdown, and so on. When playing against the Guild Dwarves in the "surprise mode", one must be wary of ALL those types of decks and prepare for all of them. Add in a couple (read ton) of potential mercenary units, and it's becoming quite impossible to plan for everything.
I don't have a good example at hand, because they are more situational, but it happens often that you plan according to a certain strategy (let's say you turtle) to avoid some trap your opponent might have if you're offensive. But it happens to be that your opponent also had this other tool to totally destroy turtles. So you're basically doomed and you have to take a chance in the strategy you adopt "in case" your opponent can't answer well. It's this luck factor I totally hate.
And if you try to play defensively (read react to your opponent), you might end up in a stalemate if your opponent tries to do the same, since both of you try not to let any opening, stick in your corner, and never summon anything to keep the surprise. And that's just boring.
What I totally enjoy from standard builds is that the decks are usually made with a couple of different tactics in mind. Taking the example of Guild Dwarves again, one might use all 3 strategies mentioned earlier and switch from one another during the game, and everybody can plan according to that because there are a finite amount of things to do.
With every faction having mercenaries (an a lot to come), their refinforcements and maybe more in the future, I'm pretty sure nobody here will ever be able to anticipate everything. With revealed decklists, the tools are fixed, and it's up to you to manage your resources correctly to deal with them all.