fair enough .. but a criticism would then be that people don't do the other stuff as they are doing this?
Well that depends. If they're not going to do more than click a link in an e-mail every couple of weeks, then they probably weren't going to be terribly effective as activists anyway.
I think one of the ideas of having this huge umbrella organisation that covers many different issues, broadly of the centre-left, is to combine forces. People who are interested in Palestine are likely also going to care about Burma, and with minimal effort can learn about the situation and lend their voices to campaigns which they might not ordinarily come across.
In some ways, whether or not it leads to more active involvement is irrelevant. No doubt sometimes it does, but avaaz is about numbers on petitions and fund-raising to place political advertisments in relevant sections of the press. It's effectively a global centre-left lobby.
I said "umbrella organisation", but it really isn't as far as I can see. It does have roots in a number of other organisations, but it doesn't seem to join forces with independent campaigns much. It would be very powerful if it used its reach to bring people's attention to the grassroots stuff that is happening around these issues. So far it seems to pursue its own line, which is broadly similar to that of other campaigns, if often less radical, but offers no direct route to finding these other campaigns.
I think it could be a lot better, but I don't think it's worth building a conspiracy theory around. The power of any individual campaign that they run is determined by the proportion of their membership that click the link or donate. If it's going to satisfy the activist urges of someone who signs up to it, they were never going to be much of an activist anyway. It's a good way of pooling resources to try and effect changes on some issues of concern to the left, and that's not a bad thing.