Adam - I mostly agree with what you're saying about efficient markets (I am the guy who publishes
generic rookie rankings based only on NFL draft position). But since disagreement is more fun, here are some reasons for thinking that the NFL draft is a little bit farther from being an efficient market than you say, and that fantasy ADP is a little bit closer.
One important feature of a market which allows it to be efficient is that people can play both sides. If you think a stock is undervalued then you can buy it, and if you think that it's overvalued than you can sell or short it. People who have information that a stock will do better than its current price suggests buy and push the price up, and people who have info that it will do worse sell/short and push the price down. If 5% of people trading stocks are idiots who buy bad stocks, they push the price of those stocks up. If no one could play the other side, then the stocks would be stuck at an overpriced price, but in the actual stockmarket the idiots just created an opportunity for savvy people to sell/short those stocks and make money. In an active market, they will quickly push the price back down to where it should be. The price reflects the combined information of everyone who is trading in the market, so it isn't thrown off too much by a few overenthuastic traders.
The NFL draft is missing that feature. It's just buy-buy-buy, with essentially no selling. If one team thinks that speedy Johnny Stonehands is a first round talent at WR, then they're liable to draft him in the first (or second). There is no way for other teams to "sell" and push his value back down to the consensus of the 32 front offices (except very slightly, by passing on him when it's their pick) - his value is determined by the team that values him most.
If we could look at the other 31 teams' draft boards and see that none of them had Stonehands graded any higher than the 4th, then I think it would be a safe bet that he will do worse than most other first round WRs. Since we can't see their draft boards, the best we can do is listen to the Waldmans, the leaks, the conventional wisdom about "reaches", and so on. It's not a lot of information, but it's something. (Note that this only works in one direction - if a player drops to the 6th round then that does basically tell you the consensus among front offices.)
Fantasy football doesn't have the same incentives (although there are some incentives - pride, small amounts of money, etc.), but ADP does have the feature of letting errors on both sides (partially) cancel out. One league may have a guy who is in love with Jason Witten and drafts him too early, and another league may be full of people who don't appreciate Witten and let him fall too far; in ADP those will cancel out. Any one particular league may be wildly inefficient (since you don't have millions of people are bidding against each other and getting bid-ask spreads down near zero), but ADP will be pretty efficient at capturing the information from lots of leagues and condensing it into a single number for each player.
Unlike the NFL draft, ADP has the weakness of relying on the judgments of lots of moderately-informed people, many of whom are using the same heuristics. That does lead to some systematic errors, including (I think) a tendency to overweight injuries (in dynasty), to overweight the importance of recent performance, and (combining the previous 2) to overly penalize players whose performance recently suffered while playing through an injury. But if you don't have one of these sort of plausible systematic reasons for why a player would be mis-valued, you should be careful about valuing a player very differently from his ADP.
(And I actually use a similar sort of reasoning with the NFL draft. If I like a player and then he gets drafted in the 6th round, I'll consider if there is a systematic reason why NFL teams might undervalue him relative to his fantasy value - e.g. character risk, injuries, limited amount of tape against good competition, etc. If there isn't - if it just turns out that none of the 32 teams liked him very much - that is a bad sign.)