Started a playthrough on "Plus" mode a couple of weeks ago, and it's certainly tougher. Several opponents will kill you in two hits, and the first staggers you so the second is hard to avoid.
The "levelling" continues to be a bit weird and annoying; the rusty piece o' crap sword I found lying about now is infinitely better than the epic, sword-master hand-crafted sword I earned ages ago.
I've also found the levels on the quests to be odd now. I'm Level 63, but all the main quests are about 5 levels lower, while the secondary quests and contracts are about my level. That means it's hard to wander about and do the secondary quests as I normally do, or else the main quests will be too easy and offer virtually no XP reward.
Overall, I've found it a tad more boring playing through again, more than I expected. I am enjoying exploring a few options that I never tried the first time. ("No, Yennifer. I don't love you any more!". Wait, crap, I thought there would be dialogue where Geralt meant that as a joke! Exit to Menu! Reload! Reload!) I'm looking forward to the Blood and Wine expansion as that's the one bit I haven't tried yet. Plus, making sure I collect all the Gwent cards, and don't wait around until one of them dies...
Now, to sit back and enjoy the sunset.