A Quasi-Manifesto
With a few (amazing) exceptions, the Half-Life 2 single player "mod scene" is dead. Releases, few and far between, often re-tread well-explored gameplay devices and themes from the original 2004 release OR seek to emulate the monolithic release schedule of the commercial game industry. HL2 modders should abandon this practice immediately.
Here are some guiding values of Radiator, collectively known as "PIES" because pies are delicious:
Provoke thought
So you made a level, that's great - but does it have a point? Do you have anything to say? Any commentary on what you perceive is a great injustice in the world? Insights on the human condition? While it may be debatable whether games are "art," they are most definitely a medium. Convey something with your work - it doesn't have to be novel or mindblowingly insightful or a coherent message, but it should at least be something.
Iterate
Yeah, it's the latest game development buzzword, but it works. Do not plan out every detail of the project; rather, design the main gameplay systems and core mechanics, produce a prototype, and gradually test and refine that prototype. It is faster, produces greater results, and keeps focus on the player's experience.
Experiment
There is no point in copying Half-Life 2 and building another map set in City 17; even Valve has grown tired of it and used Episode 1's plot to make sure it could never use City 17 again. Amateur maps should take design risks and pursue a new setting, art direction, gameplay mechanic, or (ideally) a combination of all three.
Shorten
Keep it short and (unlike the majority of mods today,) feasible. The goal is to finish your work, a problem that all of us have. To help you overcome that, remember: if you're going to reach for the stars, settle for the lowest one you can find!
Some Corollaries:
Don't cargo cult - meaning, don't have dedicated PR managers and don't focus on pumping out screenshots or weapon renders. It's a waste of time.
Don't remake your favorite game - instead, make something interesting.
Make something fun. The "art game" movement would have you believe that "fun" isn't necessary - and maybe in 50 years it won't be - but for now, fun and challenge are the primary means of engaging players. Games that straddle the middle line, between art game and mainstream, combine the best of both worlds; games like Conor O'Kane's Harpooned, for instance.
"If you don’t feel personally exposed when publishing the game, you did not make art." -- Daniel Benmergui
"Make your own kind of music \ Even if nobody else sings along." -- Mama Cass