One barrier to bentos for a lot of people might be the whole idea of eating cooked food that's cold, or at room temperature. The basic bento in Japan is meant to be eaten at room temperature, and is still very tasty (insulated/keep-hot bento containers are not that widespread in use, despite the efforts of manufacturers). Aside from some food that's designated otherwise, we are geared to thinking that food that's cooked should be hot. It's true that food that's meant to be eaten hot can taste blah when cold. There are some tricks to use when making food that you intend to eat in a non-heated bento though.

• Make sure that you flavor it well. When food gets cold, flavors can get muted. So strong flavors work better. Subtly flavored things will taste bland. Strong flavored sauces like Worcestershire sauce, steak sauce or tonkatsu sauce, ketchup, soy sauce and so on are popular for bento cooking because of this. This is also why you may feel you want to bring along some soy sauce for pouring over some of your food.
• Salt things well. Related to the above, but for the sake of flavor you should make sure your food is well salted, because salty flavor can fade away. (This is why snack foods are so high in sodium.) You may want to balance the salty foods with non-salty (like plain rice) for health reasons though.
• Avoid very greasy foods, There's nothing as unappetising as cold, congealed fat. So avoid foods that have a lot of animal fat or other fats that solidify at room temperature - butter, ghee, etc.
• If using fried foods, make sure they are well drained and cooled before packing. Deep fried foods are very popular as bento box fillers in Japan, as I've shown here previously. The key to making sure that fried foods like tempura, tonkatsu (breaded pork cutlets) and korokke (croquettes) taste good in your bento box though is to cool them down completely before packing, or the batter will get soggy and limp - not nice!
• Anything liquid is better very hot, or very cold. Lukewarm soup is not very nice! Soup should be piping hot, so bring it along in a thermos or insulated container, unless you have a microwave at work. Or bring along a mix, homemade concentrated soup or miso soup ball and add boiling water. Stews and such also do not fare well at room temperature, so you'll want to pull our your insulated lunch box. (On the other hand, stews, curries and such are some of the few foods that do taste great kept hot for some hours.)
• Make sure your pasta is al dente, and flavored and coated well. I do like pasta or noodles on occasion in my bento, but I don't like soggy or limp pasta. So I cook the pasta al dente, or just a bit chewy. And I make sure it's well coated with whatever sauce I'm using, so that it's not sticking together turning into a lump.

[Edit, added later at my mom's suggestion]

• Avoid using any sauce that you used a starchy thickening agent in: Stews, some soups, and especially gravy that have been thickened with a roux, cornstarch and so on when thickened may form a most unappetizing skin.
• Garlic and onion must be cooked well, or they will smell when cooled. Garlic and onion seem to get stronger when they cool, unlike other flavors.