In the final analysis, it's up to your players to prevent metagaming -- as you've noticed, it's easy for them to figure out what's a minion and what isn't. That said:
I like to keep monster knowledge checks in mind. I know which PCs have high Nature, Arcane, Religion, and Dungeoneering skills, and I make sure the players know the things their PCs should know. For example, I'll say "oh, and Zenja, you've studied those weird Fell Taint things," because she's got a high Arcane. I don't ask for a roll, I just provide the information. This removes a lot of the emphasis on playing metagames to figure out the best tactics.
Further, on the occasions that I don't give information, the players are more likely to accept that it's a mysterious monster. They get a bit more worried, since they know it's something special and funky.
Some questions to answer in your social contract:
Can my character die without my consent? In D&D (and most action-based games) the default answer is 'yes'. Subquestions to ask: Will I get a hint that I'm in serious danger? (In 4e you usually won't need one... it'll be obvious that you're low on surges and survivability.) How likely is this? (Players in my games know that I will kill characters, but it hasn't actually happened more than a couple of times in the last 10 years. The possibility is enough.)
Can my character die as a result of a single failure? Are there effects which kill or disable you at once if you fail a save, regardless of your state of health otherwise? In D&D 1 the answer was 'very much yes' and each edition since has been less so - a very good thing IMO. 3e in particular made a deliberate effort to reduce this, 4e more so - so in 4e you can say 'no' with no real change.
Can my character die as a result of another PCs actions? D&D mechanics assume a social contract in which the group are all on the same side, and in 4e this assumption is very strong. But good stuff can be done with characters that are mostly on the same side; conflicting goals -> character development -> entertainment. So: Might another PC abandon the group during a fight?
In particular, especially in D&D, cover can other players kill my character? as an entirely different question from whether the game world might. D&D 4e assumes 'No' as a default here, and 3e assumes 'very unlikely'.
[Edit/added, thanks to Valadil:]
Can my character die when I'm not present? If a player can't make it to the game, does their character vanish, or get played by other players, or by the GM? It's demoralizing to have a character killed or permanently damaged when you're not there. (Our group uses: GM tries not to kill the character when the player isn't there, but only if players refrain from using the PC as invulnerable point man... PC actions are by group consensus, but the PC does nothing especially heroic or dangerous. Basically, a no-score draw.)
Can my character be resurrected once dead? In D&D the default answer is 'yes, at a cost', if you're high enough level. In 4e the default answer is 'yes, at a not-high cost', especially in LFR. (Look at these rules; in a home campaign they have implications for the game world. We play LFR adventures... but the LFR resurrection rules seem too cheap to us; we multiply all costs by 10.)
Do the rest of the group have to try and resurrect me? In D&D, a regular party might even have in in-game contract for this one. Be sure it specifies who pays!
At what level does my replacement character join the group? (Our long-standing D&D and Star Wars answer has been 'at the bottom of the level below your previous character's level, plus some bonus xp based on how you roleplayed the death scene'. Another common answer is 'Same level as the lowest-level character in the group'.)
What non-base abilities/equipment can my new character have? In D&D in particular, magical items are the issue. 3e and 4e assume a certain level of magical item power for any given level of character; al cash value by level can be an acceptable guideline but not a dramatically brilliant one. In D&D for mage characters, you also need to consider 'How many spells does a new wizard know?' (in 3e) or 'How many rituals do I have?' (in 4e).
Adventurers being adventurers, and depending on your group's play style, you may also need to ask if my character dies, do the party loot the body for useful magic? (GMing note: Having an unknown relative show up and lay claim to the deceased PCs possessions is a fun way to cause trouble for item-oriented groups... especially if they had a mission-critical item at the time.)
The Living Forgotten Realms rules cover all of this well and are worth a look, but don't use them as a model for a regular group. They're designed to handle a situation in which you're playing drop-in convention games with random strangers and want the GM to recognise existing characters. They do a decent job of examining the kind of question that can come up.
Best Answer
There's no reason to employ a mechanical solution here, go with the narrative one that makes sense.
If you want to have a mechanical solution, employ a skill challenge to determine if the charges are set properly, wired together proplerly, are lit properly, ignite correctly and in sequence, and then use the results to determine how effectively destroyed the bridge is (0-1 failure it's completely gone, 2 failures it's passable, but slowly, and 3 failures it fails to detonate).
There's no need to concern yourself with anything further mechanically about the bridge as the challenge will abstract all of the elements you're concerned about with "do they succeed, and how well."