Hot sauce! Frank's Red Hot is particularly good for adding flavor and not too much heat.

Can you add some sort of side thing (rice, etc.) that also has a slight flavor to mitigate the blandness?

A smidge of ketchup and or mustard and or barbeque sauce, depending how you roll. Stir a little in before you heat it up.

worcestershire sauce and/or Bragg's Aminos.

A bag of Doritos to go along with it would be good, too.

What were the original ingredients in the stew?

Beef, carrots, potatoes, garlic, onion, water, red wine, some thyme, 2 bay leaves, a little bit of salt & pepper.

You need to add some kind of boullion or broth for flava.

Add some more pepper and some garlic salt - if you have any v8 sitting around, a little bit of that would probably go well, too.

Old Bay?

I'm gonna throw everything suggested above in! Even the Doritos! But not Old Boy, I don't have any- need to get some.

Worcestershire is your friend and so, even though it seems kinda odd, is tamari or soy sauce. Try mixing some of those in. Nutritional yeast also gives extra flavor and boost and if you have any of that, yeah. Or toss in a can of condensed french onion soup.

.salt, pepper, thyme. Lack of salt may be the major problem. You can add salt in the form of Worcester sauce, soy sauce, bouillon cube, garlic salt, etc.
.caramelized onions
.mushrooms add depth of flavor (umami)
.salsa
.wine, red, white, sherry
.a splash of cider vinegar
.paprika or thyme are good in beef soup

Any one of the suggestions about sounds good to me, though my second attempt (after salt, which you've tried) would be something sharp or acid, like a squeeze of lemon, or a little more wine (white, red, even sherry or vermouth) quickly reduced in a separate pan, or a tiny splash of good vinegar, or some tomato juice.

Sometimes a little brightness makes a HUGE difference.

Nothing to suggest, alas, but I'd love to hear whether/how the various additions improved things. I too made a huge pot of beef stew Sunday night, with a rather different problem -- way too much thyme, so the whole thing has a kind of bitter resinous flavor to it. Adding a slosh of red wine helps, but I may also try some Worcestershire or soy. (*sigh*)

I did a stew this weekend that worked out well - here's what I did:
1.5# stew meat, dredged in flour and browned in a little oil
1.5C carrots in pieces
1.5C celery in pieces
1/2 medium yellow onion, coarsely chopped
8 oz sliced brown mushrooms
a few potatoes cut up
1 sachet made of thyme and rosemary
1 C bold red wine
1 bay leaf
salt and pepper
Beef stock.

Dump the whole shebang into a crock pot, adding enough stock to raise the liquid level to about 1/2 way up the ingredients. Cook for 8 hours on low. Stir once or twice when you're not keeping the kids from destroying the house.

Oh and I think I deglazed the pan that I used to brown the meat with wine and dumped that in. Probably. The sachet of herbs made a big difference.

As for yours, pepper, garlic powder, ground rosemary, cider vinegar or red wine vinegar might help to liven it up. Malt vinegar might work too.

Ok, so last night I threw in dashes of:

Worcestershire
Soy Sauce
ketchup
balsamic vinegar
garlic powder (I don't have garlic salt, must buy ASAP!)
onion powder
kosher salt

Let's see how it is.....::slurp slurp:: Not bad! Not fabulous, but an improvement. Will continue to experiment in the coming days!

Teeps,

Next time you do any kind of stew, make sure you try 3 things:

1) roux (fat + flour in a 1:1 ratio, stirred until browned),

2) sear the beef.

3) substitute good quality canned beef stock for water

The Maillard Reaction is your very bestest friend here.

Also whatever your recipe is telling you to use for flavour / spice? sounds like you should probably increase it. I find many off-the-shelf recipes (as it were) extremely bland.

I'm too late but just had to say: smoked paprika! (Which is pretty much my answer for nearly everything...)

Of course, it will improve with a day or 2 of just hanging out, and you will be convinced we are brilliant!!

Madamina has it... although I was going to suggest Tabasco.