I played a bit of multiplayer last night. After having only leveled up to 4 or 5 prior to last night I'm at like 16 now. Some thoughts:
* As people have mentioned, a fair chunk of people online are seriously dirty racers. They fly down the straights behind you and don't even brake for the turn before ramming into you, bouncing them around the corner in more or less OK position while you ram into the outside wall. Most of the time, this catches up with them and they pay for it - they miss you once and you'll never see them again for the rest of the race. But other times you get fucked. A couple times I was in second, pretty confident about being able to catch the guy in first but racing clean so not forcing it; waiting for a good clean passing opportunity. Then out of nowhere I get taken out by some jackass and thrown into last. I get pissed and frustrated, so I abandon everything else and just try to gun down the offender and ruin his day. Probably what he wanted, but its really frustrating and not what I'm looking for in a racer.
* Lag can really kill close races in fast cars. I had a few close races in R2's and R3's where I'm drafting behind some guy at 200+, close race, clean race, etc but then his car starts bouncing around in front of me from the lag. It's really distracting and can cause unintentional wrecks. Most races I was in had some element of lag, but it was especially noticeable in the highest performance tiers.
* Car types don't appear to matter when racing on dirt - is this the case? I would try to pick AWD, higher clearance vehicles, but I don't think I get much of (if any) performance advantage over a very low clearance RWD sports car with similar ratings. So I just go with the best numbers I have, or the coolest looking car.