Stock questions. What to taste for when making stock?
Since you are going to be using it for cooking, you're absolutely right to not add salt. Especially if the cooking involves reduction, you want to be able to control the salt level at that point. Unfortunately, that does make it a bit hard to gauge the flavor of the stock. A beef stock should actually taste like beef, like if you thickened it and added salt, it would be a passable gravy.

Especially for beef stock, it helps to thoroughly roast the meat and vegetables before adding them to the water. Don't skin your onions, roasted onion skins add great flavor and color. 

If you didn't roast your ingredients before adding to the water, consider roasting them now. Fish your beef and vegetables out of the water, dry them with a paper towel, and put them in a 450°F (230°C) oven, flipping every 20 minutes or so. Meanwhile, keep the water (stock) at a simmer both for safety's sake and a bit of reduction.

Once your meat and vegetables are very brown (just this side of burnt), add them back to your stock and simmer for at least another three hours.

If possible, deglaze your roasting pan with a bit of the stock or even better, some small amount of wine. Any brown bits on your roasting pan are going to boost the flavor of your stock.

If after all of this your stock still seems flavorless, ladle a bit out and salt it just to taste (leaving the stock in the pot still unsalted).

If your salted broth still tastes too weak, yes you can reduce it. You can also add a reduced-sodium beef base like Better Than Bouillon. Doing so will add considerable salt (unfortunately reduced-sodium =/= low-sodium), so be judicious.

Like Stephie alluded to in comments, several peppercorns, a few bay leaves and some thyme couldn't hurt.