We talk a lot about good lists and good play here on the internets; there's a strong drive, at least amongst competitive players, to push everything to the limit. It isn't enough to just to have an okay list, you want the best list you can bring. Nothing wrong with that, after all; but the downside of that can be underestimating "non-optimal" lists when facing them on the tabletop. Many of these supposedly-subpar armies can be quite dangerous in their own way; for example, Daemons actually are quite beastly if they get into CC unharmed, and Footdar can be very resilient and put out surprising firepower at short ranges from Wraithguard, etc. People pick these armies because they do have strengths, even if they usually aren't as strong as their defenders will claim.
So there is a very real danger in underestimating an opponent's list just because you have dismissed it as a "real" competitor; if you don't understand the whys and hows of a list's failings as well as respect its strengths, you may very well be headed straight for that feeling of "Okay, I got this match... no wait, I don't got this match."
Understanding the opponent's list is critical against not only good lists, but also "bad" ones. (I am going to use the phrase "bad lists" a lot in this article to talk about non-optimal, poorly-designed, and otherwise subpar armies that players bring to the field for whatever reason; please don't take offense if I list your pet army here, some of my pet armies are in the same boat.) As I already said, even imperfect armies can be quite effective under the right circumstances or if dice rolls go their way; if Orks can pass a bunch of KFF saves on their Battlewagons, for example, they can put many enemies in a bad situation. If you are still stuck in the mental space of "Orks are bad, I can't lose to Orks," you probably will, because you are thinking about them in the conceptual, general sense and not what's across from you on the tabletop.
That is the important thing: play what's sitting across from you on the tabletop. Even if it's a not-good army, know what it can do and how it threatens you. Equating poor overall performance to being just poor in general is a fatal mistake, so take the time to evaluate the enemy army, figure out his threats, create a plan, and execute it just like you would against a top-tier list.
Just as importantly, remember that lists and generals are not the same. Just because the guy is piloting a crappy battleforce army doesn't mean he's bad at the game- he may have been playing that army for ten years now and know it like the back of his hand. That doesn't make the army not bad, but it DOES mean that he may very well be capable of using it to its absolute peak of effectiveness, so if you treat him like a brain-damaged fifth-grader, he might just roll your ass over. Let's repeat that again, because it's kind of important:
A bad list does not make a bad general. A good list does not make a good general.
This assumption comes up way too often on the internet; "Oh, so-and-so won with this list, that means it's really good" or "Well, I'm bringing RazorWolves, so I figure I pretty much can't lose." Bullshit, and horseshit piled on top of it with a light dressing of pigshit. There's no magical rule that forces good players to use good lists, and there's likewise no divine bolt that descends from the blue to make you a genius once to lay inspiration and aptitude at the feet of a bad player who finds a good list. Both list-writing and generalship are skills, and they are separate skills; if you want to be good at them, you'll have to practice, and not everyone is necessarily talented at both simultaneously.
I will admit, I have gotten my ass kicked by bad lists before; it happens sometimes. Occasionally, it's luck; more often, it's because I misplayed, either from inexperience or overconfidence. The former is forgivable, but for a competitive player, the latter is not. When you face someone across the table, don't half-ass it because you think they're a n00b due to the list they're bringing; give them a good fight. If it turns out you're pummeling them, let up a little, but if you walk into it smirking and assuming they're gonna be a pushover, you will deserve every inch of the beating they hand you.
