OK.. Alot of people are going to say the same things to you here. They're going to be: 1)White Raven Tactics, 2) Moment of Perfect Mind, 3) Iron Heart Surge, 4) Insightful Strike/Greater IS
Some people are going into Swooping Dragon Strike/War Master's Charge
I've been playing a Warblade for a little while, and have a few opinions myself.
1) White Raven Tactics - One of the best/the best maneuver in the book. It's pretty easily broken if you can use it multiple times a round. Basically you want to declare that a) You can't use it on yourself, and b) Someone can only benefit from one use of WRT per combat round. It's a little rough still if multiple people have access to it, but something that you can probably live with. Used properly, it's a superb teamwork utility. Helps make sure the party member who needs to go right now goes right now. The party will love it when they're fighting a bunch of undead, the cleric rolls low init. and the warblade bumps them up to the beginning for a quick turning.
2) Moment of Perfect Mind - A lot of people complain about their party's warblade never failing a will save ever. A lot of people don't understand that this uses an immediate action, which means you can't use a swift action next round to refresh maneuvers. Then you can't use a maneuver while recovering maneuvers, which depending on strictness of DM can mean that set of actions, that turn, or that round... That turn means only one will save/2 turns, that round means /3 turns. Also keep in mind that you can't take immediate actions while flatfooted, or when you're denied actions (ie. stunned, etc) Also, with the small amount of readied maneuvers... Keeping MoPM on all the time is a significant amount of resources dedicated to something that doesn't necessarily happen all that often.
3) Iron Heart Surge - I don't actually use this one myself, so alot less to say about it (I hope) Looks very poorly written, and depending on what you allow it to negate it will be significantly more/less powerful. Decide beforehand with the player what it can/can't do if they decide to take it.
4) Insightful Strike/Greater IS - Basically a full attack of damage in one standard action normally, seriously twinked can do a large amount of damage... Things to watch out for include items of +concentration and +con checks/con skills checks. Skill focus:Concentration helps, but really it pretty much ends up being d20+(5-8ish)+character level ever 2-3 rounds. It's alot in one hit, but the Barbarian is doing 2d6+(8-18ish) at least once per turn if not more, and doing it every turn. Amusingly though, if you're playing with action points... It's one way to add action points to damage, since you can spend action points on skill checks.
Hmm.. Otherwise... Swooping Dragon Strike (Mlevel 7), like Zurai said, you need to be able to jump fully over your target. Not an easy jump. Plus White Raven has a strike that doesn't give a save to stun the next level(Mlevel 8), no jump check or anything. Diamond Mind even has a sorta/kinda stun effect at Mlevel 5, granted with a Will save. And War Master's Charge is a 9th level maneuver. Thats character level 17. I wouldn't even start worrying about it unless you expect to play to that level. Even then it hurts, but so do 9th level spells
So, basically my advice in a nutshell... Decide what Iron Heart Surge does. Limit White Raven Tactics to a)not yourself and b) one/round. Be judicious with handing out +Jump items, but don't worry as much. Be really judicious with handing out +concentration items, they can hurt hard.
And very importantly, if the Warblade is rocking everyone's world at level 5, don't worry. Level 5 is just a really strong Warblade level. They get 3rd level maneuvers, while other melee guys don't quite get their 2nd attack yet. When iterative attacks start, you really have to weigh/measure whether a full attack or a strike is a better idea, especially if hasted.
Wow that was alot of words...