Arelith

Right, I'll avoid rambling and get to the crux of my suggestion:

A test server much like the HK-PGCC where you can quick test up to max level, but with areas akin to the regular servers where you can run around, adventure, level like normal if you really want to test something the whole way up. This helps for the latter part of my suggestion!

Passworded, but open for the playerbase (via the forums, logged-in members only can see) to test out upcoming mechanical changes on. A messageboard on entry to the server details recent/new changes that need to be tested before they go live on the regular servers.
Feedback either through boards on the test server - in different areas for different things, or just one/a few at entry, or forum PMs to the Dev Team. Whichever works best.

Why suggest this?
~ Familiars
~ Summons
~ Warlock/Feylock
~ Undead

All these updates have ended up having lengthy, usually-confusing and/or broken effects on the main servers. Extra updates after the fact have had to be done to fix these bugs or actually balance things (example: overpowered familiars), when they could be thrown up on the test server and addressed before they go live.
We all know players that enjoy trying to find out if they can break something, and it would be an easier way to work out balance before just throwing it onto the regular servers where it can be quite disruptive. It would also be a lot easier to log everything that happens on the server, given otherwise you have to dig through everything and everyone on the main.

We won't get useful feedback on game balance from a limited test. You'll notice that we usually make balance changes after a couple of months or so of exposure on the live server (and sometimes longer) - it's a question of sample size.

We actually have a test server, it's just not advertised and only runs when I need to investigate something. If I'm doing a particularly large and scary change (like, say, Warlocks) I get the mechanics straight on the test server before pushing it live.

These days, I do very little testing of minor updates or bug fixes. It's time consuming and less fun; the trade off for you guys is that you get more updates, at the cost of dealing with more bugs.