Of course, if the PCs don't have any ideas in mind it can get tricky. Draw their attention to the long planks. Let them see one on the floor below through the gaps. Allow them an Idea roll (Knowledge Engineering or Dungeoneering roll, Intelligence + Relevant skill roll) to point out certain avenues if they get stuck. Phrase it in a certain way: "The weak floorboards are a gap between you and your goal" so that the problem is defined in such a way that crossing the gap seems like the best option, in case the players get confused because they think of the weak floorboards as a sign of a No Go Area or a puzzle where they need to find the solid ones.
Be willing to get creative with these obstacles as well. Just look at those elements in Sandtrap where you can't step on the sand or else risk being attacked. You therefore have to manipulate the environment around you so that you can go from object to object without, ideally, hitting the sand at any point.
|"Wait, did he just talk? Nope, that was a grunt."|
A campaign based around Half Life, or including elements of it, should appeal to Tacticians and Action Heroes because of the use of obstacles, setpiece locations and a sense of 'flow'. You go where you need to go because it feels natural. This leads to a very streamlined and elegant gameplay experience where you get to feel clever without ever feeling frustrated. Explorers would be more likely to notice that the game is on rails as even if you allow them to tackle the obstacles in any sort of ingenious way, these players are likely to run to the edges of the game world simply to learn something new about one particular aspect or another. This will require a lot of bread crumbs to lure them back onto the path so that they don't become too obsessed with learning more about the Combine Soldiers. They're here to explore the dystopic universe and see all the sights as well. On the plus side, so long as your itinerary of locations are varied and interesting enough, they should be quite happy.
Investigators will be doubly troublesome due to their tendency to focus on gathering information on whatever piques their curiosity. They also tend to be relatively combat shy (especially when it comes to confronting superior forces) and may go to great lengths to avoid combat and therefore find different ways to attempt to tackle the enemy Combine. After all, they're more motivated by an intellectual game. If you can insinuate that the answers to one riddle or another are behind those enemy soldiers, however (such as if you put recordings on their bodies as loot) then you can bet your bottom dollar you'll get Investigators more interested.
Communicators, as always, are generally more interested in the complex psychologies of other places and other times. They will want to immerse themselves in that world and experience what it would be like to actually live there. This might slow down the game a bit as they might be loathe to leave the resistance behind and might be quite happy to simply wander the streets of City 17 to see how people live their lives. Couple them with Explorers and you'll certainly want to do some significant worldbuilding to ensure you can quickly generate the sort of locations and people they are looking for.