3 years sounds like a lot, but the amount of detail that goes into games these days is insane. The more sophisticated the graphics become, the longer it takes to develop a mod, this is just obvious.
Proof of that is, in HL1, after just a few months of the SDK's release, we already had CS and many other good mods. Now it's been a year and a half since HL2's SDK was released, and most of the more high profile mods are still under development. I think that by the end of this year we're finally gonna get the first wave of the more professional mods.
Releasing half-assed mods and updating it based on player feedback doesn't cut it anymore these days. The first release must be something awesome that grabs everyone's attention. If you release a mod with average textures, average models, average maps, average gameplay and whatnot, people aren't even going to bother with the mod, no matter how much you improve it later. We live in the eye candy age. If it's not pretty, people aren't going to play it, simple as that.
Look at Dystopia for example: one of the coolest concepts I've ever seen in a game, I was DYING to play this mod. And then they released their first beta. It was ok I guess, but it wasn't "oooohhhhhhhhhh awesome". It felt unpolished, like it needed more work. It turns out that I played it 2 times and never bothered with it again. I might be spoiled or something, but I'm sure I'm way more open minded about mods and willing to take the time to download and try them then 90% of your average gamer.
And when you consider the size of these mod teams, it's really no wonder a mod takes more than 3 years to complete.
That said, I had never even checked the off-limits website before. The mod looks great, really professional work. The basic gameplay idea also sounds very original, I hope it turns out to be some great fun. I'll definately check it out when it comes out.