At the moment I can think of 6 things besides unique species characteristics. Please note though that it will help you to consider exactly what species you want to focus on to see if individual species have more useful unique characteristics. Monkey have hands; mole (ok insectivore) burrow; etc.
Number: Because plant life is much easier to produce en mass than meat, expect that the biomass of herbivores will greatly exceed that of carnivores. For every lion, you need many zebras. They are very good at stampeding.
Size: While not the majority, the most giant beasts tend to be herbivores. Ignoring elephants and rhinos for now, there are large numbers of large grazing mammals such as bulls that can do some serious damage in they put their weight into it.
Repurposed defense: Herbivores are evolved to survive carnivore attacks. Obvious examples include horns and hooves but for strategic purposes, poisons or spines could theoretically serve a purpose. If they are good enough to defend, they are usually good enough for attacking. They could also help to slow advancing forces by standing their ground.
Repurposed escape: Some herbivores survive not by defending themselves but by getting away. This means they might be good for reconnaissance but are generally outclassed as far as this goes by falcons and what not. They may, however, be able to move quickly and not be seen. A mouse is great at hiding and spying. A sparrow can get in and out quietly and quickly to pass the info on.
Incite riots: Finally, you can use the carnivore/herbivore thing to your advantage. While the pure carnivore enemies plan their war, the more allied forces sneak into enemy territory and speak to the other herbivores. "Hey dude, I'm like you but it is better over where I'm at. Help us out and it will be better here." Let the monkey and hare then release all the cattle. The rooster than calls loud enough to signal the other camp to go forward and all hell breaks loose.
-added later-
- Easier food supply for managing army: How are creatures that can't use tools going to store food for an extended conflict? Squirrels are smart enough to store provisions. I think human intellegence level cows can find a way to place a few haystacks in sheltered areas and have sheep resupply the pile as they can. I have no idea how a wolf could do that with meat if they can't freeze or salt it. Wars are won by logistics as much as if not more so than tactics. A large army of herbivores definitely stand a better chance on the logistics side as long as they can protect their dead (and admittedly feed it to allies carnivores). Before long, the enemy army will bring potential future allies (see insight riot) to just behind the front lines for food. OTherwise, internal conflict and consumption will ensue. The carnivore army can best survive if it is advancing (pillaging) or being attacked. If it is sieged or forced to retreat it cannot recover as easily. The only advantage they have logistically is that their food can walk. (Note this idea works better if someone is smart enough to find a way to carry hay well such as tying it somehow to the sheep's fleece.)