What is the incentive to collect transactions?

Adding transactions to the block you're working on will slow down your generation rate. What prevents the majority of generating nodes from ignoring broadcasted transactions and making the network unreliable?

I don't know if there are any technological safeguards to prevent this, but there is a social one: if you drop transactions, others might drop yours.

Bitcoin supports transaction fees paid to the node that records a transaction into the block chain. If too many nodes start dropping transactions, you can pay a fee when you want to increase the likeliness of your transaction being recorded in the next block.

So would it be smarter to only process transactions which profit you? That way if you want to send money you need to include a courier fee or nobody will confirm it. That seems reasonable to me, where you can add a fee yourself to each transaction, and people can configure a threshold of how much of a fee they expect before accepting a transaction.

That's a clever (and very free-market) solution. How does BitCoin currently deal with transactions that aren't published in a block for a long time? Is there any risk of them being lost?

Are there any estimates on the average transaction fee once users' incentive to support the network via bitcoin generation is mostly dried up? How does this scale with the number of users, the size of network, and the total transaction rate?