I add tomatoes to my soup after the lentils are cooked. I also add thyme and marjoram.

Garlic?
I make mine w/ curry powder & bay leaf - & cayenne pepper too - we like things spicy here.

a little shot of vinegar, or some mustard gives it some distinctive flavor.

Vegetarian lentil soup: saute onion, carrot, celery, garlic, and potato in olive oil. Cook the lentils in salted water with a couple of bay leaves and another herb: I usually use oregano or fresh parsley. You can add a single clove and some freshly ground black pepper if you want. Make sure there's enough salt.
To save time usually I cook the lentils in one pot and saute the vegetables in another, then combine them, but you can saute the vegetables then add water, salt, lentils, herbs. I occasionally use tomatoes in this soup.
Or you can try cooking the lentils in an Indian dal recipe: the one in the 'Vegetarian Epicure Book 2' is good.

I think your problem was no salt! What colour lentils were you using?

I usually make a veggie version, but when I do add a meat, it's always a ham bone or smoked sausage, like kielbasa. Never beef.

I think that if you make it like a split pea or navy bean soup, you'll see the possibilities. Celery, carrots, onions, garlic and a meaty ham bone. Tomato juice gives it a nice flavor, as does a few dashes of Tabasco.

Lentils don't take any time at all to cook.

Lentils need a bitter counterpart. I use tomatoes and/or spinach, and also always include a bay leaf. Other tangy options are red wine or vinegar, but don't add all of these, depends on your vision for the overall flavor profile. Also don't skimp on the salt. Suggested seasonings have been mentioned here, but I would add oregano to the list of possible good choices. Bay leaf is pretty much required for good soup, IMHO. With a beef broth oriented soup, sauteed portabella or cremini mushrooms might be good, or shitake, if you like either of those. The plain white ones don't have much flavor.

This is a lovely simple soup with lentils and chard:

Here is a link that might be useful: Lentil and swiss chard soup from Food and Wine

I believe Barb is restricting her sodium consumption, so adding more salt may not be an option. I make a Greek-style low sodium lentil soup frequently. I always add a can of either unsalted tomato sauce or diced tomatoes, along with a bay leaf, carrots, onions, garlic, & celery. I add thyme or oregano too and often a good handful of chopped parsley. Always a lot of freshly ground black pepper.

I use unsalted chicken broth and drizzle in 1-2 teaspoons of vinegar too. The vinegar seems to add a bit of sparkle. If the tomatoes seem too sour, you could add a couple of pinches of sugar to mellow things out. Also a couple of glugs of good tasting olive oil.

I don't use plain beef because it is pretty bland in that lentil mix. Even though I'm aiming for a low sodium soup, I'll often cut up a favorite sausage link (only one) from a local butcher -- very thin slices. That means that just about every spoonful gets a bit of sausage but there's not enough overall to make it a sodium-laden dish.

And if I'm making a sausage-free version, I'll often use regular canned tomatoes. That bit of salt from the canned tomatoes really helps. However, I don't add salt or use salted broth.

I've never met a legume I didn't love...but depending on how you cook them, they can be absolutely delicious, or bland, bland, bland. In addition to, or instead of, the flavorings other posters have suggested, freshly grated ginger pairs well with legumes. Another way of adding flavor is to caramelize your onions before adding them.

Around the World, there are many different ways to prepare them. But, there is often, no matter the method, herbs, spices involved, a 'finishing touch.'

A drizzle of olive oil & vinegar...a squeeze of lemon, a dollop of sour cream...or in Indian cuisine, spices toasted in clarified butter. I use the clarified butter, (ghee,) sometimes, but other times I use olive oil...mustard & cumin seeds, heated until they start to pop, and then poured into the soup. Very often, I also briefly saute garlic in the same oil, (once the mustard seeds start popping,) or, asafoetida, if available.

If you like cilantro, it compliments any sort of legume, chopped and stirred in at the last minute.

I've actually stirred salsa into lentil soup, when I've been in a hurry.

Basically, you want something sour, or acid to 'brighten,' the flavor of the legumes...to counteract the starchiness, in addition to whatever other ingredients you add for flavor.

Keep trying. Legumes are a wonderfully nutritious, healthy, and inexpensive addition to your diet...and they can be very yummy, once you get the knack. :)