Here's a simple scenario that describes several traps to get things back on topic.
There is a locked iron door between you and where you need to go. You don't have the key and have no idea how you could get it.
Barring specific class abilities, there are three main methods of dealing with this problem - pick the lock, break the door in one shot, and break the door via HP damage.
Pick the lock: The system heavily nudges you towards this one because it's a locked door, you're supposed to interact with the lock right? No, that's a trap. Since good locks are cheap, the locks on anything important will be well out of reach of your lockpicking abilities. Since it's an iron door it's clearly blocking something important. This is a means of wasting skill points on a skill that does nothing and that's all.
Break the door in one shot: This requires a DC 28 check that is Str + size + other benefits. Essentially if you are anything other than a Dungeoncrasher Fighter you're not breaking it down, even if you're strong it's not likely at the levels this constitutes a significant obstable. If you are you're still not that likely to break it down unless you stack many different things that boost Str checks in addition to the +10 bonus from Dungeoncrasher levels. This isn't necessarily a trap, as you made that investment for Dungeoncrashing and breaking doors is just a random bonus but if you're anything else trying is a trap as you're only going to make a lot of noise and alert anything around to your presence and general intentions. The system does kind of nudge you towards this one, but not really.
Break the door via HP damage: It has hardness 10 and 60 HP which is decent at these levels. However it's an inanimate object. You can PA your entire BAB, use any take accuracy penalties for more attacks and so forth and still hit so breaking it down is very easy. The system nudges you away from this one slightly. Despite it being clearly the best option as it works reliably without specific investment - any melee character can do it.
What you are saying is true, from a certain point of view. [/Obi-Wan Kenobi]
While everything you say is accurate, it is only valid if you are attempting to solve the problem "linearly" but a huge aspect of 3.5 is that non-linear solutions generally exist and sometimes are more optimal.
For your example, it might also be possible to hide and wait for a guard charge, to find the owner of the key and beat him up, use alternate movement modes like burrow, climb or flight, or comedy option: knock on door and say in orcish "Pizza Delivery!"
This is part of what can make it hard for beginners is that sometimes existent linear solutions are worse than non-linear ones and at other times non-linear solutions are wasteful compared to simple solutions. Evaluating this takes a level of skill mastery that many beginners don't possess and can render something as simple as a locked door a major puzzle to them while an advanced player would just power attack the damn thing.
this is not a trap beginners should be warned of. some of the core classes (bard, cleric, druid, wizard, sorcerer) are very good in providing a servicable chassis for a successful adventurer.
I would actually say the Bard and Sorcerer are often traps for beginners. Sorcerers can easily suck with bad spell selection and Bards can become very unfocused in the hands of novices.
Barbarians are a good class for beginners in campaigns that end before leaving the middle levels because you have a very simple strategy of hulk-out, charge and power attack. They can teach tactical positioning, character focus and begin to show characters the mundane/magic imbalance when you reach middle levels.
I think a big thing is that it is less important for a beginner to actually succeed completely than it is for them to learn how to succeed. Lets face it, nearly everyone's first character kinda sucks, but as more experienced players you want to teach them how to make their next character not suck as much and so on until they create good characters by a natural progression of skill.
