A lot can go wrong actually.
1) the tank gets silly ideas and use riot blade combo only. For DPS.
2) The tank is not confident in MP management to the point that they are quick to use riot blade to stay above 85% when they never get below 90% long before the fight is even over (or at the start of the fight)
3) The tank gets silly ideas and instead of using flash multiple times, loses agro on the main target and more because he kept switching targets to strike them individually.
4) Follows the "flash twice" trend like a robot; fails to understand more has to be done, especially when a (competent) summoner is there and/or when nobody is marking.
5) Fails to understand that switching targets constantly confuses DPS.
6) Fails to understand provoke is not safe to pull with because it does nothing more than facepull. Shield lob to get a better hold on things.
7) I've seen one newbie go through the manor at level 29 not knowing she had Rage of Halone, and nobody(cept me) bothered to try to get her to do it right.
8) Summoners always AoE
9) Fast forward to 50, and look at that they don't know to use flash AND circle of scorn together.
10) Then watch as their only claim to fame never be used, ever.
11) Fails to understand power is needed. Not for DPS, but for threat. Otherwise, someone else is going to be the tank.
12) Fails to understand just how much of the staying alive part is the healer. Especially when a black mage (or someone else) is the tank.
13) Takes things for granted only to be killed by the Ascian right at triple. Fails to learn from his mistake and gets creamed again for the same reason. Everytime I've been there, the paladin dies because of this.
14) Takes things for granted.
15) Rather than Shield lobbing, they will instead run in to finish a combo. Resulting in a messy pull.
16) Takes things for granted, never really understanding the DPS's part in this. Good DPS, unfortunately make your threat an issue. Incompetent DPS, makes your long and easy. If you are underpowered, then both ways end with a slower run, because the Bad DPS are slow and the Good DPS gets mauled.
17) Fails to understand how Provoke works. You don't just use it and think it's the end of that.
18) Remember when some silly fools con'd some poor PLDs into thinking riot blade is a DPS combo and use for more damage? Fast forward to 60, and look at that, they're using RA instead of halone and they wonder why the DRG is tanking the tank's target.
19) Cross Class mayhem
20) You have the lowest threat multipliers of all tanks. Not because you deal weak damage. Get that silly thought out your head. All other tanks have better threat multipliers than you.
21) NOW think about how much stronger they are to you.
22) Inept at AoE damage before 50. After 50, you move up to practically inept at AoE damage.
23) Sword Oath. Look at everyone else.
That's not all. But I'm done writing this list. Also Sword Oath.
It's not to say a lot of warriors and dark knight don't do a lot of the same things.