(Warning, long post ahead!)
F1 - for me it has lost it's charm. Even 10 years ago, when racing was arguably not great, it held more appeal to me. We had yelling V10s, and cars looked like pure and simple speed machines, clean and concise. Like the Renault R26:
Another issue is F1's (and especially Bernie's) lingering elitist attitude they are better than everyone else, because they are Formula One. F1 isn't purely about the racing, but primarily the show. Now a great championship can have both, but with F1 it has appeared that the show has taken over. It's more obsessed with the glamour and locations than racing. Abu Dhabi, Valencia, Singapore, all events which on a postcard look absolutely phenomenal, but do they really have great race tracks? It's financial based - Autosport readers will be familiar with Jim Bambers caracture of Bernie with the $ signs on his glasses, which is actually quite accurate.
Techonology wise F1 has also tried to outdo all. It's undeniable that F1 cars always have been and remain phenomenal pieces of technology. How KERS, DRS, and how the engineers build these cars in fairly constrictive regulations is incredible, but to the average joe wanting who switches on, maybe not a motorsports fan, they just want to see a race. They don't want to have a lesson in how energy is recuperated from braking energy and stored.
For the WRC, i've seen it's recent fall. My very first taste of WRC was the live Wales Rally GB finale on ITV4, when Petter Solberg, Carlos Sainz, Marcus Gronholm, Tommi Makined, and Sebastian Loeb engaged in a 4 way fight for the WRC title. It was riveting and gripping. High profile stars, big, noisy mud flinging cars, cars with a profile - we all know our Lancers, Impreza's, and Peugeot had a high profile then. Citroen's domination certainly pushed people out, but you cannot blame them. They where simply great and no one else could consistently hold a candle - Peugeot where strategically withdrawn by the PSA group who saw no sense in two brands from the same stable competing against each other (Peugeot / Citroen) Mitsubishi left, Subaru faded away, Ford would go on with a slightly dog-like Focus.
When we're all discussing this, we have to remember as well the economic crisis from circa 2008-2011 (correct me if memories hazy). When something like this is happening, financing a motor racing programme is not top priority, so they get the cut first. I like how WRC has seemingly picked up its spark again, TV coverage is better now (if still minimal), and makes are slowly coming back. M Sport have had enough money to run a competitive Ford Fiesta package, Citroen arguably have not been full strength for a year or two. VW have come in and done what Citroen did and gone all out attack. Whilst it's not too competitive right now, at least WRC is amassing the interest. Toyota are coming back soon as well, and the R5 regulations that the FIA have drawn up i've read about provide it a real chance to grow.
BTCC is blossoming. After Super Touring burnt itself out, grids were low and sparse. I think it was 2005 at Donington we had a grid of 12/14 cars - 3 Vauxhalls, couple SEAT's, couple MG's, couple Civics, and 2 Lexus IS200's (thinking the HPI entries.. Williams / Curely IIRC? Anyway I regress). But BTCC simply used the new FIA rules to its advantage (S2000) and gradually got grids going up to the point of the viability of having there own rules formula - the NGTC. From a couple back in 2011 (The Audi A4 and Toyota Avensis) to 31 in 2013 says volumes. It's also had a stable, accessible slot with coverage pretty much unrivalled to any series broadcast in the UK, maybe not counting Sky Sports and the F1 coverage recently (not got Sky Sports however)
Indycar has potential but it is wasted. Stuffing everything in a 6 month season is great whilst it lasts, but it means the off season is so long it drops away from public eye constantly. Whereas NASCAR with it's massive schedule is always out there, so the public in the US see it. However I think the only good move NASCAR have made is the Gen6 in recent years. The chase is a gimmick, and it's over complicating affairs. The racing as well... I don't blame Hendrick Domination or Jnr not going anywhere fast. It's how NASCAR made things so artificial with the debris cautions, the GWC was a good idea but 3 attempts is just asking for wrecks.
Reminds me of the USCR. I have very few good words about that series. Terrible race control, poor driving standards, it's got none of the pure drama of the ALMS, down to the fact people in race control believe races need to be bunched all together with wave arounds, pitting in packs and keeping cars compressed. For proper sportscar racing you want the WEC, not without flaws but drivers, cars, reams, and racing is of a much higher quality. Blancpain has become the No.1 GT series in the world as well, I mean 60 cars all in the same class (GT3) at Spa last year for the 24H is proof of it booming.