I keep seeing people ask about these things, so I'll summarize. A couple of weeks ago I made several changes to address forum slowness.

1. You now must be logged in to search.
2. action=.xml was totally disabled. I'm seeing some people request that it be reenabled, so I may bring it back up with stricter limits later.
3. action=printpage will no longer work if the topic has more than 500 posts. You'll get an error. If you used this for easily searching topics, note that the upper-right search box will search only the current thread if you're in a thread.
4. The 5-board limit on searching was removed.

In this and all instances where I remove functionality or impose limits, if you are prevented from doing something which should really be OK, you should describe your use-case in detail in a post to Meta (or a reply here). I may make changes.