The ones that really get my blood up though are the mods that have an amazing gameplay demo video on the site, but they have not released any playable content after a huge amount of development time.
The old rule "release early, release often" holds true for games as much as any other software product. Initial betas of counterstrike were playable, but clunky. Still the team knew they had something fun that people wanted to play. Thats all you need! The community stuck with them because it was always fun to play.
Many mod teams have an unhealthy obsession with perfection or that someone will steal their ideas. They should probably realise that if your attepting perfection, you wont reach it and... trust me.. your ideas probably are not that original and it's doubtful there are lots of people out there just waiting to steal your mod content. If you feel your idea is original, its probably not that good. There are very few original good ideas around. You are not going to re-invent gaming as we know it and at the end of the day, its just a feckin computer game.
It's also very unlikely that your amazing mod, when finally released, will get you hired by a games company. Its far more likely that, by releasing actual playable content iteratively and regularly, you'll learn a lot more through player feedback and become a better game maker thus making you far more hireable. You'll also have a much higher profile out there if people are playing your stuff.
A strange confusion around the difference between a story and a game also seems to exist. I've seen so many mod pages or blog posts where people say, "we've almost got the story finished so we can start building soon".
Story means feck all when building games. Its a veneer painted over gameplay to make the whole thing more pallettable. It's a delivery package for gameplay but it should always come secondary to gameplay.
Finally I don't understand why weapon models seem to be the first thing that mod teams focus on. Its almost always the first sneek peek you get of any mod, most of which dont make it. There must be a humungous arsenal of weaponry out there that never saw the light of day...