My real reason for writing this is that a lot of people in the beta seemed to treat the server as an extended vacation from the live realms, and this upset a lot of old PTR hands who assumed people knew they were supposed to submit feedback, and not just play with all the cool new toys. Yes, you should have fun, and you can contemplate the trippy philosophical notion of your character's existence in an alternate universe (duuuuuuuuuuuuuuude!), but you can do a lot of things to help patch 3.1 launch without issues. The point of the test realms' existence is for Blizzard to test everything new with as large a population of players as possible. If you're uninterested in submitting polite, honest, and frequent feedback, you're making it harder for them to get an accurate sense of the patch's impact on live realms.
If you find a bug, reporting it is good. Being able to reproduce it is even better.
It's hard to get an accurate sense of the seriousness and frequency of a bug unless everyone who encounters it reports in. Blizzard tests internally, but it's not always possible to extrapolate results to a population of 11 million players. Even the PTR isn't always a good statistical sample, but you can help improve that by submitting bug reports whenever you encounter one, and providing as much information as possible on the bug's context and (most importantly) whether you can reproduce it. The inability to reproduce a bug makes figuring out what's causing it a lot harder.
If you don't like something, say so and give reasons. If you like something, say so and give reasons. If you're indifferent, say so and give reasons.
This is a little more true of individual boss encounters and raids because then there's a unified something on which to write feedback, but it's still true for the rest of the PTR as well. Writing "This encounter sucks!" on the feedback form and clicking the submit button is not quality feedback. Writing "This encounter is amazing!" on the feedback form and clicking the submit button is not quality feedback. Writing nothing on the feedback form because you really don't care is even worse than hating or loving it. At least with the first two Blizzard can get a rough sense of whether people like or don't like the encounter, even if they'll have no clue why people feel the way they do.
If you don't like something: Why didn't you like it? Did it seem needlessly difficult or convoluted for certain classes and specs? Are there factors that might have made it harder than it had to be that Blizzard can't actually control for (e.g. a raid with players in Naxx-10 gear trying Ulduar-25)? Or was it just plain boring or too easy?
If you like something: Was the encounter totally awesome? Was it made with 100% pure win sauce drizzled over an entree of amazingness and served with balsamic woot? What element or elements made it especially fun? Does it feel tuned correctly? Did it seem undertuned for your group in good gear? Do you think a raid group in gear similar to, better than, or worse than yours would have a different experience?
Or did you not really feel strongly either way?: Even if you fall into the third group, write something honest. Do any classes or class abilities seem very overpowered, necessary, or undesirable for success in the encounter? Does trash respawn time seem reasonable? Does the amount of trash seem appropriate for the size and length of the instance? Does the quality of itemization seem commensurate with the difficulty of the encounters?
Technical problems are functionally the same as bugs. If a lot of people have issues, they need to know about it.
Everybody's system is different, but there may be consistent complaints concerning bad framerates, lag, or disconnects on certain encounters. This is not an infrequent problem on instance servers that get overrun (see: Naxxramas, popularity thereof) and/or encounters with heavy AoE damage. If this seriously impacts the viability of an encounter or achievements associated with it, write in.
On that note, the test realms are not always meant to run smoothly.
Part of the process involves stress-testing to see how many characters a zone or server can realistically support before the game is basically unplayable. It sucks, but you may very well be one of the players unwittingly testing this. Cowboy up, and submit feedback when gameplay is rough. If a few nights of nasty lag ensure a more smoothly-running game when 3.1 goes live, Blizzard did its job and you did yours.
A note concerning the 3.1 PTR: not everything is going to be available all the time.
Certain boss encounters in Ulduar will only be available in the EU, certain ones will only be available on North American servers, and for the moment none will be accessible when the encounter team isn't around to watch what's happening. Don't count on being able to schedule consistent, unchanging raids if you manage to get a raid team imported.
And as a final note, don't get too attached to anything you see on a test realm.
Blizzard may be trying out the most awesome, unbelievable, jaw-droppingly overpowered ability of all time and holy s#^$# you can't believe it's actually in the game and it's so much fun and it's amazing....and the next day it might be gone. To take some creative license with an old saying, every skill and talent is perfect until it makes contact with an 11-million count playerbase. There's always some weasel out there who will figure out some diabolical means of using an ability or item in a fashion other than intended. Sans weasels, sometimes new stuff just doesn't work, so out it goes. Plenty of stuff hits the PTR that I suspect the developers are either divided or ambivalent on, just to get some sense of how it would play out on a live server. No matter how badass your PTR Ulduar raid stategy is, or how cool your talent changes, etc. -- don't toss a fit if it gets axed.