Plate is hardly the super-armor some folks think it is. If you're interested in exactly how armor rating works (well beyond the scope of this post) you can check out the ratings Blizzard is using now at WoWWiki to get an idea of how armor caps out. Essentially, the most physical damage reduction you can get from armor is 75%, which is a problem more for druids than warriors since druids hit the cap a lot faster. Yes, I'm telling you that druids have higher armor than warriors despite the fact that warriors wear plate. If you don't believe me, feel free to go to the druid forums and ask them.
There's not a DPS warrior in the game who hasn't worn mail or even leather in his time. The first DPSer I ever knew as a warrior was a gnome on Azjol-Nerub, back in the day when you could take an entire 40 man raid group through Blackrock Depths and into Molten Core via the portal there. He was a gnome in a mix of leather and mail because there just wasn't much better for him back then. Back when AQ40 and Naxx were the big time, I wore Scaled Sand Reaver Leggings forever because there just wasn't anything else dropping for me at the time. Thankfully, there are more DPS options for plate nowadays.
DPS plate generally has lower armor than equivalent tanking plate. For an example, here are two blue gloves to consider, the Felsteel vs. the Ragesteel. The Felsteel's are item level 114 while the Ragesteel's are item level 115, making them roughly equivalent. As you can see, the Felsteel's are designed for tanking with higher armor, defense, some stam and customization options via their sockets. The Ragesteel, on the other hand, are a pure DPS piece and have inferior armor (admittedly still higher than mail or leather) and stats aimed at increasing your damage output. For comparison, we'll look at the High Warlord's Plate Gauntlets, also at item level 115. Since these are PvP gloves, they appear as a mixture of the two, with stamina slightly superior to the Felsteel but without the socket customization options or armor and DPS inferior to the Ragesteel but with the resilience and stamina helpful to staying alive longer to make use of that damage.
One of the big mistakes a lot of warriors make when they hit 40 is training up plate and immediately hitting the auction house and buying armor for every slot, replacing often superior blues they have from crafting and instancing just because those items are mail. Don't make that mistake - wait to switch your mail out until you actually have an upgrade to go into that slot. This is especially true if you've been farming SM for the mail Herod drops or the gloves from Mograine - don't be quick to toss those aside for inferior greens just because they're plate greens.
Now that I've gone on and on about some of the negative aspects of plate, it's time to admit the positives. First off, it is the highest armor you can get if you can't turn into a bear. Higher armor generally means you can grind faster since you take less damage from your average non-caster mob. Plate, it also must be admitted, looks pretty awesome most of the time. I mean, take a look at this, or this or even this from the old Naxx days which really needs to come back into the game somehow. Also, there's a heck of a lot of well itemized plate out for whatever you want to do, be it tanking, PvP or PvE DPS. And the only people you'll have to roll against for it or outbid are other warriors and paladins.
Okay, since it would be mean to talk this much about plate and not list a few pieces, here's some stuff you should be looking for as you hit 40 and want to start trading up. Keep in mind that a lot of this gear is improved in 2.3, and Wowhead's already showing you the new stats so be sure you don't go farming for a piece you'll outgrow before it updates.
Carapace of Tuten'kash - drops in Razorfen Downs. This is a really good chestplate for a new 40, it has a good mix of stats although the armor's kind of low (compare it to a level 31 crafted blue, the Green Iron Hauberk) and you can farm it easily if you're of a mind to as the boss that drops it is not that far into the run.
Horned Viking Helmet (Horde Only) - drops in Uldaman. Eric the Swift, the mob who drops this hat, is friendly to Alliance so many of us miss out on this sweet level 40 hat with a very nice use ability as well as good stats. I'm still not sure it's worth trading in the Raging Berserker's Helm from SM for it, though.
Icemetal Barbute - drops in Razorfen Downs. Ammenar the Coldbringer drops this hat, with good stats for DPS or tanking (although as many comments indicate, the Raging Berserker's is still better from a pure DPS standpoint) so if you're there for the chestplate anyway, might as well try and grab this.
Plated Fist of Hakoo - World Drop BoE. Good stats for a newly 40 warrior who doesn't know yet if he's going DPS or tank or wants to do both.
Skullplate Bracers - drops in Uldaman. Several mobs there drop these nice bracers which are about to get nicer in 2.3.
Obsidian Greaves - World Drop BoE. My first warrior to 40 had these in the bank when another set dropped while he was farming for mount money. It was a nice surprise. Decent blue boots you can pick up at the AH and which have a nice spread of stats for DPS or tanking at that level.
Enormous Ogre Belt - World Drop BoE. Outside of running Arathi Basin this is probably the best belt you're likely to get at 40. I'm surprised the spirit hasn't been taken off of it yet, but as it's BoE they might well miss it. It's still worth picking up if you can get it affordably.
Golem Shard Leggings - World Drop BoE. If you're level 41 and are looking for tanking blues or slightly more AP than the Scarlet Leggings off of Herod and Mograine, then these are basically your option. I don't know if the 4 AP you'd get over the Scarlet Leggings is worth the 10 stamina you'd lose if you're using them to DPS or PvP in.
Big Bad Pauldrons - drops in Zul'Farrak. The bad news is, you'll have to wait until level 45 to equip these. The good news is, they're getting a nice DPS buff in 2.3. If you can't get these you can pick up the Wyrmslayer Spaulders, a BoE shoulder set, from your local AH, but as of 2.3 the Big Bad's are purely better for DPS and can be worn a level earlier.
That should be enough talk about plate. Next week, I have no idea what I'm going to talk about. I guess we'll find out together.