My answers in bold.
I'm new to Bitcoin and spent some time to read through the informations on this site and play around with the client. Now I've got a few questions/suggestions now and hope to get some replies.
1. Why isn't the wallet encrypted by default? A lot of effort was put into making transactions secure, but anybody who has your wallet has your money. It should be trivial to add a strong encryption so that you can only open a wallet when you enter a password. Then people would not have to use eg Truecrypt for that. It would also be a good idea to confirm each transaction with the password, like you do with the normal online banking systems.Wallet encryption is coming in the next version
2. Are messages added to a transaction public? Or could I use this message to send back confidental information, like passwords or private links?Everything in a transaction is public, so if you put a message in a transaction it will become public too. Please note that (standard) transactions sent to other bitcoin addresses can't have messages included with them. Transactions to ip-addresses can contain messages, but those are disabled.
3. Can I invalidate an address I have so that every transaction to it gets rejected? Those addresses could then also be purged from the blockchain to keep it at a reasonable size and only need to be stored in some sort of rejectchain. Perhaps the client could even suggest to purge addresses which have not been used in any transaction for x months. That way you could sort out addresses you never need anymore and make sure nobody accidentally still sends money to them.This is not (currently) possible. Every address you generate with the bitcoin client will be stored in your wallet.dat file, so if someone sends you bitcoins to an old address you will still receive them.
Old addresses can be purged from the blockchain if they don't hold any bitcoins anymore, but this is not implemented yet (there are some topics about this, but I didn't read those in details to see how it would work)
4. Are collisions possible? It works for MD5 and SHA1, so is it theoretically possible that someone creates a new address which is already used by someone else and that way also gets access to his bitcoins?Collisions are possible in theory, but in practice it's so improbable that we'll probably see the end of the universe before this happens.
5. Can wallets be merged? I could set up a miner on another system and now I'm asking myself how to join both wallets later. I know I can just transfer the coins, but don't you also get fees for the blocks you've mined? How can that be transfered?Wallets can be merged just the way you suggested: by transferring all the bitcoins from one wallet to another. The fees you gain from mining are just like normal bitcoins that you can transfer, so there is nothing special about those.
6. How can I display smaller fractions in the client? I've seen people discuss 0.0000001btc transactions, but I only see 2 decimals.The display will adjusts itself
7. Can't the Windows release be compiled as static? I have an older system where the current release (0.3.23) won't start (some error about freeaddrinfo); client 0.3.20.2 still works however. Only problem with that version is that the background of the "transfer/addresses" block shows the underlying window, as if it's transparent when switching to it.I'm not familiar with Windows, but you might want to report this as an issue on github
Tips are welcome: 1HGvvuyYoRHmXM2VQZdQvVaCSCnPKRArZS
Hope those help!