Much has been made of the comparisons between Death Knights and Warriors. Tobold initially said that he he would retire his warrior because DK's are more fun to solo, but has since conceded that he would not invite a DK to be a main tank. (I'm not sure if Blizzard has actually balanced for threat/DPS levels yet at this stage in beta, so that situation may change.)
I happen to be in the position of having a level 61 warrior in early Outland to compare to my Death Knight, and my initial impression agreed with Tobold's. The DK seemed much much more durable (indeed, Blizzard has begun to nerf Blood spec to compensate), capable of killing multiple, tougher mobs. Part of that is due to gear; DK's start with armor that's better than the early Hellfire Peninsula quest rewards, and indeed my DK is slowing down somewhat now that she's moved on. Then I took my warrior out for a spin and I noticed a big part of the difference.
Warriors need to hit things
That observation may sound kind of obvious, but it goes into the difference between rage and runes/runic power. Melee damage in WoW (regardless of class) is divided into so-called "white damage", the damage generated by your autoattacks and displayed in white numbers, and special attacks, which do damage displayed using yellow numbers. Special attacks have an inherently higher hit rate, which makes sense because you're spending resources to make the attack. The result is that, if your weapon skill/hit rating is too low (e.g. because you recently switched weapons and are 15 skill points behind) compared to the enemy, your white attacks will start missing before your specials do.
The reason why this matters is that warriors need to hit stuff in order to get rage to fuel their special attacks. By comparison, Death Knights can miss every single auto attack and they will still get runes and runic power to work with. The result is that the DK is much better able to handle foes that are slightly over their head. Of course, the DK also has significantly better AoE abilities than a Fury warrior does, and their healing capacity with at least a few points spent on Blood talents is substantially higher, but I was surprised to rediscover that the warrior can kill almost as quickly as long as I'm dealing with foes I can reliably hit with autoattacks.
The implication for warriors, unfortunately, is that they need to be cautious about overextending into content tougher than they are, at least until they can get a decent amount of hit rating. Casters are immediately considered whatever level they are for the purposes of spell hit calculations from the moment they ding. Other melee classes do also have to deal with the handicap of being at least five skill points below their current cap at every level up, but it feels to me like the warrior is especially hindered because of their dependence for rage.