Substituting fresh vegetables/fruits for canned
Tomatoes are one of the few things I use from cans, mostly because they have more flavor than the ones you usually get at the grocery store, but about 2-3 good-sized, ripe tomatoes substitute just fine for a can.

The canning process includes heating the food being canned, so using fresh fruits and veggies might require more cooking to get the same effect or texture in a dish, and as was noted in the comments, could also cause some other effects like denaturing enzymes. If you're preparing a recipe that doesn't require you to cook the canned veggies, you may want to steam or blanch their fresh counterparts before adding to the recipe.

I find the can also affects the flavor, but not in a good way, so I wouldn't worry about that difference ;-)