Really, the aspects most relevant to the market success of the new Canon camera are the ones which haven't been commented on much - card slots, shutter modes, AF performance, intelligent AF modes, buffer, video output and lens availability at launch and within the first year or so.
The exact specifications of the sensor don't actually matter all that much. Whether it's 24MP or 28MP, it's not a megapixel monster - it's not competing against the A7r3, D850 or Z7 for ultimate image quality or base-ISO output. It just needs to be a decent all-rounder, performing well at ISO 6400 as well as ISO 200. And almost any sensor Canon could possibly put out should meet the necessary standard.
Most of the exact details of the RF mount also don't particularly matter. No matter what it is - even if it's physically identical to the EF mount - you're going to be up for new lenses. Same applies if you move to Nikon or Sony, so there's no disadvantage to Canon there - if you want to move from SLR to mirrorless and take full advantage of the technology, you'll need new lenses. The one thing that would make a difference is if the new mount can actually accept EF-mount lenses without an adapter. That would allow Canon to design all future EF-mount lenses with linear stepper motors, allowing equal use on both SLR and mirrorless bodies, giving Canon a major advantage during the transition period (before the AF performance of their mirrorless bodies can match the best of their SLRs).
But these other factors will determine which section of the market it suits, how well it fulfils its role and whether it provides a compelling reason to switch now (as opposed to waiting 2-3 generations).
Dual card slots mean the difference between a camera suitable for professional use and one that's not. Even if it's a one-in-a-million chance of failure, if you miss someone's wedding or lose half the photos at a function, you're not getting hired again (and could be up for a large chunk of money). And many non-pros also don't appreciate spending $15k and several weeks' leave to go shooting photos somewhere exotic, only to come back with nothing. Considering that the Sony bodies have two slots and the new Nikons only have one, this will be a major factor in determining where the Canon fits within the mirrorless ecosystem.
AF performance is the second big factor. Is it at the level of the A9/A7r3/A7III, or more like the A7II/A7r2 (as the Z6/Z7 are supposed to be)? Can it match the 5D4 AF-wise, or will an early adopter be sacrificing AF performance compared to an SLR, even using native mirrorless lenses? This will be one of the big factors determining whether it makes sense for people to switch to Canon mirrorless now or whether it makes sense to wait another generation or two, since SLR would still give better performance and they'd get little out of switching except a smaller, less-capable body. Intelligent AF modes - face detection, eye focus, etc. - will be equally important. At 24-28MP, and without long telephotos to go with it, this is neither a wildlife camera nor a landscape/non-action camera. A lot of the time, it will be used to take photos of people - portraits, events, weddings, concerts, etc. At the moment, eye focus gives Sony a major trump card over SLRs in these settings. Whether Canon can match this could mean the difference between having a real contender for for that purpose, or just another camera that's almost an 'SLR with an EVF'.
Video output will be another big factor. This is a general-purpose camera, not a dedicated megapixel monster. Shooting events (whether in amateur or professional hands), it will likely be shooting a combination of stills and video - event and wedding photographers are increasingly doing both as part of combined multimedia packages. And mirrorless cameras, much more than any SLR, are well-suited to capturing video - a mirrorless camera is effectively a video camera that shoots extremely high-resolution video at an extremely slow frame rate, with EVFs, focusing systems, etc. all being the same. The new Nikons appear to have a very capable log format - can Canon match this?
Finally, the lenses available at launch, and within the first year of release (after 1 year, you may as well wait for the next generation of bodies), will have a major impact on uptake. Is Canon going to release slower, smaller lenses, indicating a focus on the downsizing and light-travelling crowd (with serious photographers expected to either stick with SLR or go for Sony), or f/2.8 zooms and f/1.4 primes, which would indicate a focus on capturing the higher-end market before Nikon can?
