I agree with everything you said. If someone is brand new to Assetto Corsa and they ask about mods, I always recommend they start at Race Department because of the organization...there are usually screenshots or even video clips showing off the mod and when it comes to cars, the mod creator usually is trying their best to make the car drive like the real thing. If they're making a 1971 Vette, for instance, I expect softer springs than you'd find on a 2015 Vette with some body roll. I'd expect it to be slower than a modern sporty sedan with brakes that aren't very impressive compared to today's cars.
Yes, crap mods do end up on RD, but I think it's easier to discern what looks & sounds worth downloading. I checked that 1979 Interlagos - the track is 9.9MB in size. Right there, I would expect a completely $hit mod. Oh yeah, I just remembered...if you look at the date on that mod: June 18, 2014 with nary an update -- I would not expect that track to be even remotely good. You're also 100% correct regarding the mod 5-star rating system. I'm going to try and remember the term you used, an encouragement system, when discussing this in the future because that's exactly what it's turned into and that is bad for all of us. I'll admit, I've probably been guilty of it a couple of times. Now I just won't even bother giving it a rating unless I love the mod. Without giving the mod creator's name, I one time gave a 3-star review to a license plate mod. They were California plates from the USA except the plates were still the size of the European plates that are default on AC cars. It stretched out the letters & numbers and just looked like crap. I got a PM from the mod creator going bull$hoot on me saying that it deserves better than 3 stars even after admitting that the size isn't correct at all and the texture of letters & numbers was terribly stretched. I stuck to my guns. About 3 or 4 months later I deleted the 3-star review, of my own volition, not because he asked. So, I have been part of the problem at times...not part of the solution.
The majority of my mods come from sources other than RD as I'm sure yours do to. It took me some time, but I can pretty much tell you if a car mod is going to be really good, really bad, or decent enough to hold onto based on the name of the modder and the version number they're applying to the mod. That's one of the things I really respect about Assetto Land. They make sure to give proper credit to the mod creators. There are too many FB groups that leech cars from Velos Mods, AC Mod Central 24/7, etc. and re-upload onto DepositFiles.com servers hoping to make a few bucks off the hard work of others.