A few thoughts after a few hours:
Contrary to outward appearance, the handling seems finer, easier to control. It's still a bit twitchy and hard to pick a line, especially at low speed, but improvement is improvement. The visual feedback in first-person, no-helmet view is very TT like and that I like. Wheelieing is more fun because you can see where you're going now.
Still very engaging physics (pro, no aids save auto tuck), but, because of the more stable controls, it feels a little more filtered and staid. What this implies is that it was always heavily filtered, but the controls were just wayward before. They need now to inject some life and broader feedback into that virtual rider to create a better sense of the dynamics at play.
Sounds are improved:
There are now better samples from better recordings - these are a bit "sample-y" sounding, like GT (think idle transitions). Some bikes sound incorrect, though, and there are still no modified sounds. But it's a good start.
Still no doppler in replays, no details like wind noise, tyre noise, chain noise etc. Very sparse soundscape.
The source selection, balance and mixing is still way off - most of the time you can hear your own bike when racing (which is I suppose an improvement) but in replays it's a total cacophony. Also bikes behind you don't scale in volume properly with distance.
Banal music again; now with stuttering issues! The menu sound effects are infuriatingly repetitive and cannot be turned off.
If you switch the menu music off, the little incidental videos are silent.
Menus are improved, but the core loop still has needlessly tedious button presses for confirmations and lots of mini pauses for loading / saving / synching. Main loading times are massively improved as others have said. I had issues with network timeouts, too, due to not being logged into PSN; you get a lot of hangs until you set synching to manual, which you have to do each boot and only after the first failed synch.
Customising:
No more separate icons for wheel colours etc.
Prices are all over the place: 2000 credits for a brake disc but only 500 for full carbon lace-up wheels?
Rear brake lines can now be coloured too (but not on all bikes), as can handlebars.
Chains improve torque, but not power?
The World Tour is pretty badly maimed I think, at least going by what's available at the start, plus one of the middle tiers. Gone are the sensible categories like triples and twins, replaced by things like "street style" and broad events like "nakeds"?! I don't quite understand the significance of the seasons (after 8 races, you start a new season), and there's some weirdness with the invitationals (I couldn't continue with the World Tour until I did one). I think it's intended to add variety, but the races are basically the same just with easier AI and bigger payouts - perhaps people ignored them in the first game.
There are sometimes far too many bikes available to use in the races, and the performance variance, as mentioned, is huge. I guess this is so you aren't funneled by your first bike choice so much. The bikes you draw depend on what you enter, but there's no real balance and there are lots of "rabbits". The PP system is clearly broken; you can annihilate the likes of an F800R with an XJR1300 on a fast track (predictable from the power figures), but the Yam loses out to the Beemer by around 50 PP! That was after my Duke 690 got annihilated despite having a slightly higher PP. Best to use power and weight figures, plus knowledge of the track, as before. The problem is that events have a PP limit now, and going in blind is not fun...
Tyre and AI skill choices help a lot to balance your personal difficulty, unfortunately by trial and error almost per event category. The credit bonus for optional difficulty is much appreciated - I'm currently running at 21% which makes up for my slow progress through infrequent wins! (Medium AI).
I chose the Ducati as my first bike, because it's a V-Twin; I got told I favoured style above all else, which I resented at first. Then I proceeded to put shiny bits on it. Perhaps she has a point, I am a tart after all!
Overall it's much more polished and the core gameplay (riding bikes) is fun enough to hold my attention, easily replacing the first game. Although the familiar grind and "what next" feeling from the first game has resurfaced here (made worse I think by the arguably nonsensical structure of the World Tour), I think it's just about a worthy sequel for the better controls, sound and general polish.
However, compared with the first game, the sequel's World Tour does very little to educate about types of bikes and their traits as compared with the first game. It's like: "hey look: bikes, go nuts!" instead of creating an implicit understanding (though hands-on experience) of why they're all so different and why manufacturers make certain decisions about packaging and engine configuration.
It's certainly a much better prospect for anyone new to the idea or who passed on the first game, but I think they failed to make it as interesting and engaging for the long term (save adding new content...)