Won't start. I've replaced the fuel filter, the spark plugs, wires, and ignition coils. Will run off of starting fluid, but not by itself. Checked fuel pressure and it's coming up around 40. What else could be stopping it from starting?

You need 3 thing to get an engine to start. Fuel, air and spark. remove any one and you get a No Start.
No start even with starter fluid suggests there is no spark to ignite the fluid. Try going back to basics. Check the silly, obvious stuff first. Yes, I've been caught out by these,
Is the battery fully charged? is the air filter clean? Is there fuel in the tank? Have you checked for spark at the actual spark plugs?
When you crank the engine what exactly does it do? This will give clues as to the cause. If it just turns as it cranks suggests no spark, Trying to start but not running suggests fuel supply.
- Check you have spark all the way from the spark plug to the coil pack and power to the coil pack itself.
- Is there fuel getting to the cylinders?
- Is the air intake system clear and not blocked.

BAD COIL OR IGNITION MODULE INSIDE DISTRIBUTOR RUN OHM TESTS TO VERIFY

Ok then.

Faulty fuel pressure regulator.

If you replaced the injector after the plugsthey may be fuel fouled.Pull the plugs and clean them.(you can sandblast them,or use a method I used to use-shoot a little starting fluid on them and use a lighter to burn off the contamination of too much fuel).I can't believe GM kept using this stupid design so long-I remember nearly new Vortec V6s with this same problem in the '90s.

You need 3 things for your vehicle to start and run.. Fuel, spark and air. If you've recently replaced the fuel filter I'd have the fuel pumps electrical connections checked out. It could possibly be on its way out. Also check all spark plugs, wires and coil pack to make sure nothing is damaged on the "spark system". Then of course make sure the filter is clean and that all of your sensors monitoring the air flow into the throttle body are connected. From personal experience though, I'd say it sound like the fuel pump isn't always doing what it should upon initial start up and that's where I'd direct my attention.

Get some engine starter spray. Follow instructions on spraying into intake. This will serve as a fuel source to test spark/ignition. You will want help cranking the car while you spray. If car stumbles and tries to run, you have a fuel problem. If after trying for a few minutes there is no change, then I would bet you don't have spark. Of course you could go further and say there could be a timing issue, no compression, etc. But I would start with the easy stuff first.
When was the last time you did a tune-up? Are the plugs in good shape?

Check for a blue snapping spark on 2 or 3 plug wires(or all of them). If you get good spark, then ignition is likely not the problem. A weak or sporadic spark could be a bad coil or coil pack, or a bad ignition module (the ignitor).Check the coils with a volt-ohmmeter per the specs for your car. The ignitor can be tested at a parts store.
If spark is good, check fuel filter, fuel pressure, check timing marks on crank pulley and cam marks, Try a little starting fluid in the air intake and see if it runs for a moment. If so, then it's likely a fuel delivery problem. If no change, then it's likely an ignition circuit problem.
Good luck.

Are the plugs wet with fuel?
Check for spark. No spark? it may be the ignition module.

Plugs dry and spark present? No fuel?Spray a little starting fluid and see if it will run on that.
Then check pump fuses and for fuel pump noise.

First spray starting fluid in the air filter. If it starts it is a fuel problem. If it doesn't start then pull one of the spark plugs and see if there is spark. If there is not spark then it is an ignition problem.
If it is a fuel problem and the fuel pump is running when you turn the key on, then check fuel pressure. It should be over 35psi.
If it an ignition problem start by checking fire at coil.