I'll chuck in my opinion here, for what it's worth.
Far too many people start a mod without any idea of how much work is involved. Even if you do mange to merge a great concept with a dedicated, talented and well lead team you're a fair way from success. You've got to generate interest and deal with inevitable loss of staff and resources along the way. More importanly you've got to know when to relase and when to hold back and polish. Too many mods die because people feel like they always want to add more and end up stalling and going kaput. Then there's the opposite problem, releasing some cruddy ass bug-ridden, one map, two weapon alpha that makes your name shit in the community.
Even with focused and mature leadership things can get to the point where you're too close to hte project and it makes it hard to judge when the sweet spot is for release.
Another problem is that of far too many talented people being tied up in dead end mods that their mates are running and then whatever resources they have go to waste.
As for choosing a mod team, well thats tricky and depends on what you're doing and how much you want to be involved. Some of time, freelancing is the option, contribute your stuff to an existing mod and get your name in the credits without any of the internal strife. Getting in at the ground floor as it were can be excellent, your opinion will probably count for more and you get to see the mod develop and (hopefully) flourish. It does however commit you to rather more than churning out content because there will be inevitable debates about features and suchlike, even if you've got a concrete design document.
bleh, too tired to write more.