For a quick example of the question posed; say there was a series of spiked mashers randomly coming down from the ceiling during combat between the PCs and monsters. Would the PC's be making dexterity checks or acrobatics checks to dodge the masher plates trying to squish them? Would I need to come up with an attack bonus for the trap and roll against the Reflexes of the characters? And lastly, what about the monsters in the room with the PCs during the hypothetical combat, would I need to calculate the skill modifier (in this case Dex+1/2 Level), use a straight Dex check, or roll against the reflexes with an attack from the traps?
[RPG] When is it appropriate to use Ability checks over Skill checks
dnd-4e
Related Solutions
Rules Compendium page 128 states that group checks are almost always against an easy DC and if at least half the group succeeds the whole group succeeds. Otherwise the group fails.
Aid Another is not broken...
That perception check example you give, its supposed to work that way 100 percent. Conceivably the players could also just stand in a room and continue to roll perception checks until they get a number they like. If the situation has no inherent danger punishing a failed skill roll should have no consequences.
A creature can affect a particular check only once using the aid another action. However, up to four creatures can use aid another to affect a single check. In certain circumstances, the DM might decide that only fewer than four creatures—or even no creatures—can try to aid a check. For instance, it is unlikely that more than one creature can assist in picking a lock.
From the Compendium all 5 of the other party members can't help at once and there are some guidelines for limiting it further due to physical necessity (you can't have 5 people trying to pick a lock at the same time, for example).
Also, in case you missed it, Aid Another's difficulty is:
DC: The assisting creature makes a skill check or an ability check against a DC equal to 10 + one-half its level.
Don't modify the core rules of Aid Another
At best a player can only add +2 if their role is successful and they could in fact make it harder by (-1) if they fail. Don't increase or decrease the bonuses. You can however restrict availability as you see fit and also dictate which skills or Stats are applicable to help out.
Create skill challenges & skill checks in combat that force different players to do different things at the same time
A classic example would be needing to close a portal to hell while hellish minions spew out of it. Anyone in the party can take a standard action to try to do an arcana check to start closing the gate, but then they aren't attacking. Likewise anyone who is aiding them is also loosing out on their standard action.
The pure skill challenge might consist of a situation where all of the PCs need to be doing something to solve the problem and their are real consequences on the line. Perhaps the inn caught fire and now they are trying to rescue everyone inside, have players take turns 1 at a time describing what they are doing to try to help and increment against them the effects of the fire as things get worse. Failure may mean they save one less person, take damage, or valuable items are lost to the flames, but helping someone else will cost you a turn in the skill challenge.
Best Answer
To answer your first question:
The one that comes immediately to mind is that breaking down a door is a strength check rather than Athletics. Most of the time your characters should be rolling skill checks not ability checks. This rewards the skill training and bonus system. Keep in mind that the RC has the updated DC table and that the DMG has outdated advice on how to use the DC table (including adding 5 when rolling skills which is just silly).
Now to address your specific instance.
Use a trap. Traps typically act in one of the following ways:
Like a character in the initiative order, they attack on their turn if a character is in range.
Using opportunity or immediate actions. This means they can either fire once per turn or once per round if their trigger conditions are met.
Traps make attacks, have DCs to detect and disarm and can be attacked ( inanimate objects have AC/Ref/Fort, but not Will and are immune to attacks against will).
If the traps are indiscriminate then they will need to attack the monsters, if the monsters know about the traps and how to avoid them, they might get a bonus. If the traps are controlled by an intelligent source then they might not attack the monsters.
If your traps are outside of combat then you might present them as a skill challenge, in this case have your characters use their skills not their abilities to avoid the challenges.