These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Don't forget Little Sister by Queens of the Stone Age or Club Foot by Kasabin, which fit the atmosphere of 3 perfectly.
Demonic Spiders: The police. In earlier games, they merely rammed you around and impeded your race progress, but in LA they can actually bring you to a stop and take you out of the race by placing the Player Character under arrest.
Game Breaker: The Kawasaki ZX-14 is unlocked when you reach Champion rank. Fully modified, it will blow the doors off many A-class exotics for between half and one-tenth of the price, depending on your choice of comparison. It can make short work of the difficult AI (see Nintendo Hard below). The South Central Pack (which is included in the Complete Edition) also has the Ducati 1098, which is slightly cheaper and even faster stock. Combine either of those with Level 3 Agro and they're almost unstoppable.
Goddamned Traffic: There is nothing more aggravating than having a clear shot at the finish line only to have your run rudely interrupted by a hapless AI commuter. Which is why Rockstar made Aggro.