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?

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.

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?

The premise is false.  Adding more transactions to the block you're working on does NOT slow down your generation rate.  When generate is scanning hashes, it only hashes the header of the block, which is constant size.  The header contains a hash of the transactions (the Merkle root) and is only updated occasionally.

If necessary I can write code to make nodes prefer not to use a block if it doesn't contain enough of the transactions they know about.  A discouraged block would almost always fail to be included in the main chain, but would be accepted if it did get in.  I doubt this will be necessary, since there's no real advantage for nodes not to include all transactions.