Level reuse guidelines

I thought I'd go into a bit more detail on the subject of level reuse and cover some rough guidelines on things that work and things that don't. I'll use some examples to compare and contrast different examples where possible - as level reuse has been present in games for quite some time.

Don't reuse plot critical areas
This is Dragon Age 2's main failing. Areas that are home to crucial areas of the plot are locations the player has visited potentially dozens of times before as meaningless and nondescript places  used for sidequests. It ruins any sense of exploration and the sense that something new is occurring, because it's simply reminding the player of the sidequest they did 30 minutes ago... and 30 minutes before that... and 30 minutes before that...  Alternatively, if you consider Mass Effect 1, you'll see that while it potentially featured just as much level reuse as Dragon Age 2, all of the generic levels (with slightly different arrangements of crates) were for side-quests. Optional content that while interesting, wasn't part of the main plot and climatic events of the game. This is why I found the original Halo dull after the halfway point, because the second half of the game was mostly just traveling in reverse of what you'd already done.

Use it as background
Like Sundermount in Dragon Age 2, try to use larger (outdoor) areas as different components. Dragon Age Origins did this with a number of its random encounter areas - several were part of the one "map", but the player could only see a small part of the map at any given section. The additional areas were merely background - area that the player could see and made their current surroundings real, but not accessible to walk. By doing this, none of the effort of level design is wasted (because all of the walkable area is used), but because the different component areas form the vista (partially) visible from the others, each area will feel unique rather than feeling like the same cookie cutter areas. Of course, if it's combined with the above point, then you're even more off the hook.

You saw that wall and mountain in the background from a few different areas

Limit reuse
This might seem like a no-brainer, but it warrants saying. Don't re-use levels too much. If player keep seeing exactly the same terrain over and over again, they're going to get bored. It doesn't matter how pretty it is, they're going to find it tedious. When we're talking a single player experience, especially a narrative based experience, you have to present the player with something new, because otherwise it doesn't feel to them like the surroundings are progressing with the story. Even if the setting is set in the same location over a long period of time, it should undergo cosmetic changes to demonstrate the effect of previous events within the story.

Reuse segments, not everything
Wherever possible, try to avoid reusing an entire level wholesale. If you can fragment a level and potentially present the player with those segments in a different order (or even leaving out bits entirely), then the player will likely see the deception, but is more likely to accept it as a reasonable effort to keep things interesting. Missions within sandbox games (e.g. GTA series) do this brilliant, as do many driving games. They have the player travel through part of a level they've already seen, then introduce new segments that they haven't seen. Designers can also get the player to come through the area from a different angle; have them go backwards through the level instead of fowards. Dragon Age 2 attempted to do this to a degree, but due to failiings in the above areas, it's reuse still grated on players.

This outdoor setting was used in its entirety every time

So there's a few simple pointers in terms of how to manage level reuse in games. Reuse isn't always bad, but if you're reusing levels, make sure that it is a conscious choice and reasoned decision to reuse an area rather than simply "we didn't have the time/budget to do a new level." If you're doing that, then you're game already contains one design flaw that could have been avoided.