What percent of these new currencies fail?

Very few cryptocurrencies fail, most just go "under the radar", becoming a haven for CPU miners, due to GPU miners running off chasing the next dream.

The sites that pretend to tell you which coins are most profitable to mine probably help that along too, since basically they cannot help lying to you if they look at the difficulty (which often does not actually change that often) and compare it to the price on some exchange(s).

For one thing the difficulty does not actually tell you how many coins you will get, instead it tells you whether a whole bunch of other miners are going to imagine they will get lots of coins and thus all gangbang it at once so they all get a whole lot less coins. Very likely those sites work best if you take a contrarian approach, because when difficulty rises lots of miners will think that means less coins and so leave.

Thus it is very likely that higher difficulty means more coins per hash because of less miners bothering to mine at that high a difficulty, and low difficulty means less coins per hash because far far more miners jump aboard at that low a hash.

Meanwhile if exchange rates go up, it is the people who already have the coin that get to sell it at the good prices; by the time you have switched over and spent time mining the supposedly better profit coin chances are good that people who already mined some have made use of the good prices.

Basically don't believe all the hype. A "dead" coin is, most of the time, a coin ripe for months and months and months of CPU mining, nice and easy and cheap to pick up, like BBQcoin was for so many months and like Tenebrix and GeistGeld still are.

About the only coins I can think of off hand that actually died are Solidcoin and Fairbrix, in both cases maybe largely because they cannot be compiled. (Solidcoin because source code was not released, Fairbrix because the code is so ancient it no longer compiles.)

All the other supposedly-dead coins I can think of offhand are still humming along nicely, many of them providing a nice easy steady stream of coins for CPU miners, though of course the ones that can be merged mined tend to be a little harder to CPU mine because they are included in my merged mining which usually had a whole 5870 pointed at it and is now back to having two 5870s pointed at it.

Basically a coin that has no well-known and web-based "exchange" tends to be of little use to people blowing lots of money on rigs and electricity as they need to keep dumping all their coins to pay their bills. So they go elsewhere, leaving such coins to those whose electricity is already part of their normal household budget and whose computer is already paid for thus can mine at low difficulty month after month after month in no hurry whatsoever to cash in the coins they mine.

I have been selling BBQcoin lately for a bitcoin per 10,000 coins now that GPU miners have returned to mining it and it is thus too difficult for me to mine. So those long months of quietly sharing it with other CPU miners are starting to pay off lately...

-MarkM-