I'm picking up on these key issues for people.
1. Cost ( which either is an issue or not for a person)
2. The tire model
3. The ability to try out a track or car before buying it.
4. Not knowing which track/car combinations are popular at the moment, to help select tracks and cars that you can actually use.
Does that just about cover it?
I would alter the 4th issue slightly. There are quite a few cars that have such a limited user base as to be a viable option only to extreme enthusiasts/optimists hoping that the series suddenly gets popular.
There are several "free" cars and tracks, so you can always be assured of a race in an MX-5 or a Street Stock Oval racer. The Solstice runs in multi-class races and is either at "free" tracks, or possibly it is the majority of the time. The Spec Racer Ford is free, and is often at "free" tracks and every track introduced to a season remains on the schedule for 3 consecutive seasons before being rotated out. So you can run the SRF relatively cheaply. Warning: The SRF is a handful!
If you are splashing out on a three month + Sub then you wouldn't go far wrong splashing out on the Skip Barber: the races nearly always split, they are hourly, and again, many of the tracks on the schedule will be the "free" ones. Unlike the SRF, it is a relatively friendly car to drive.
I can't speak much about the Oval side of iRacing; I would like to get into it more, but there are so many tracks (and so many similar series) that the cost is more than I can swallow. (£5.00 per corner!*) Especially as so many of the oval venues are so similar - straights are a slightly different length, banking is a slightly different angle, maybe put a small kink in one of the straights. Yeah, I sound like an ignorant European.
For an american based company, I had expected more from the subscription to drag me into oval stuff.
*or £2.50 if you consider an oval track to have four corners; I don't.
I like iRacing, but currently not subbed. If I was faster, I wouldn't play anything else, but at the shallow end of the talent pool, the differences in laps times can easily be 5-10% between fastest and slowest - leading to lonely races in the middle of the pack (I can hit the middle because 1/3rd of the field are likely to push too hard and bin it on the first couple of laps.)
For reference, I am normally comfortable playing the likes of AC and rFactor at about 88% opponent skill level. Sometimes winning, sometimes outclassed, but normally able to race for a few laps.
If I could lift myself to about a 95% skill level in those games, iRacing would probably be much more enjoyable for me.
TL
R Pretty much as you state it. If you wan't to race anything other than GT3 or MX-5, you are going to spend most of the week waiting for a single high SoF race (Strength of field when all the big boys and girls turn up to play.)
The good news is that 5 minutes on their forums will uncover when these races take place. And you will need that time for practice so that you can memorise when the car is going to oversteer/understeer - because with the current physics, if you wait to feel the slide, it's already too late.