I'd be interested to hear others' unpopular opinions in this thread. Here's a few of mine to get us started:
I don't like this new wave of overpriced overdrive pedals that simply pop out and accentuate the 1.6 KHz frequency. (Grind, Horizon Drive, 33, etc.) - I personally don't think they sound good through any guitar in D standard or higher tuned, and they only seem to work with deeply detuned guitars. Give me a tubescreamer variant any day over those, they're just not my cup of tea.
I think mic recording is NOT overrated. I've been using IRs for 10ish years now, and they're amazing and the only way I can record anymore. Having said that, a miked up amp blasting a 57 and a 121, properly set-up of course, and pushed through a tube pre and a 1176, will crush a recording done with IRs.
Not sure if this is unpopular, but I wish EL84s would stop being made. I love the idea of mini amps, lunchbox amps, but the only ones I've really jived with have bigger tubes in them. The MT15, with 2x6L6 and a smaller transformer is the model to emulate IMO in the lunchbox space.