Absolutely none
Relatively few good options are alignment-locked; the best that comes to mind, a paladin/warlock (warlocks are a Cha-using class, paladins get divine grace, eh, kinda OK), already could have been done with a variant paladin. And even barring that, you could do hexblade/warlock; arcane resistance is significantly weaker than divine grace, but it is similar, and you could stick out hexblade long enough to get the (incredibly awesome) dark companion (Player’s Handbook II).
In my games, we remove mechanical alignment entirely, so warlocks in my games never have to worry about alignment. Warlocks are a relatively weak class (Tier 4), so really there’s just not much to worry about. The best things a warlock can do (and these are mutually exclusive) are
a hellfire glaivelock (using the hellfire warlock prestige class in Fiendish Codex II and the eldritch glaive blast shape in Dragon Magic), which deals fairly large amounts of quite-reliable, very-accurate melee damage, or
going for a master craftsman capable of creating any magic item (via the chameleon’s “floating” bonus feat that can be changed daily to get Item-Creation feats, and the 12th-level warlock feature, Imbue Item)
The problem is that melee damage is more easily and effectively done by überchargers for similar amounts of optimization, and the item-crafting thing, though much more powerful than the glaive thing, is done massively more easily by just being an artificer, which get to do that trick from 1st instead of 14th (and have tons of other benefits besides).
Neither of these options benefits much from allowing non-evil lawful warlocks, non-chaotic good warlocks, or true neutral warlocks. The glaivelock, I suppose, could go with a couple levels of paladin to boost his saves, but glaivelocks don’t usually go for high Charisma, and at any rate there are variant paladins or hexblade for warlock-compatible alignments that still have divine grace.
It also matches the warlock description better than the actual rule
The description of warlocks in Complete Arcane seem to be very... inconsistent. The class features and the alignment requirements all focus very hard on fiendish sources for their power... but the actual descriptions mention the existence of warlocks with powers stemming from places other than the Lower Planes. Fey, for instance, are explicitly mentioned, and while you could argue that this is what the chaotic options are for, not all fey are chaotic and at any rate a warlock can be several generations removed from any interaction with fey. And options beyond fey and fiends are indicated.
Unlike the paladin, who literally gets his powers from lawfully serving Good, the warlock has alignment restrictions.... because an author thought it sounded appropriate. An author who, apparently, was unaware of some of the things others working on the class were saying. There is nothing to be gained in keeping the restriction.
Warlocks don’t even necessarily know who or what their patron is, if they even have one
Warlocks have power in them, and that may have come from a pact that they have made personally, or that a notorious ancestor made, but just as easily it could have been a pact made so long ago, or in such secrecy, that the warlock has absolutely no idea where the power comes from. Unlike with clerics and their gods, there is no direct link between the warlock and wherever their power came from: warlocks are gifted or sold their power, as a one-time deal, no take-backs on either side. That power may last a long time, extending even to successive generations, but it is not something continually extended to the warlock: that power is now theirs. (One might assume that a power capable of granting warlock status could take the effort to remove it, but even if you do, it would take a conscious effort, rather than simply no longer empowering him as a god would with a cleric, and it would apparently be an extreme, rare, and difficult thing to do, since Complete Arcane makes no mention of such a possibility.)
So a warlock isn’t necessarily in any kind of relationship with the fey or fiendish or whatever other powers who are responsible for their powers. They may not even be aware of them. There may not even be anyone “responsible.” The warlock class is excellent at modeling all sorts of characters, who potentially have no “patron” at all. The power could be a random fluke, a genetic mutation, the result of exposure to some secret arcane experiment, or what have you. I have played games with warforged warlocks for whom eldritch blast was really just laser beam, a magical blast built into their construction and powered by the same power source as the rest of their bodies.
And these things are all the more reason to drop the alignment restrictions on the class.
