I do a mix of on and off-road riding, but mainly on road (country roads etc.). I'm currently using Panaracer Razer XC tyres and, despite initial worries that the extremely widely spaced knobs would make things dicey on tarmac they've actually turned out to be great, much better than the conventional semi-slicks I was using (the knobs are very shallow which probably makes a difference).

I've not really ridden them on wet tarmac before today but started to notice when braking that the rear wheel seemed to skid more easily than in the dry (I generally brake with both levers equally). Now this could have been part psychological, I'm sure someone on here mentioned that they hadn't liked their Razers on wet tarmac, so I may have been a bit nervous to start with. What I did notice however was that I had no skidding with the front wheel which has a bigger rotor and is less weighted than the rear and I would assume therefore more likely to slip in the wet.

Anyway, my questions, am I imagining/over-playing things or are the Razers not a good tyre for wet tarmac and what actually makes a tyre grippy on wet tarmac, is it the tread pattern, tyre compound or a mixture of both?

The best bike tyres for wet tarmac are complete slicks with soft compound rubber.

I think on the road it's about tyre compound and the size of the contact patch – the latter not being big with knobbly tyres on tarmac

I find it's more like what makes a tyre un-grippy? If the road surface is slimy in any way, diesel being a favourite, then you're off! Most tyres grip on tarmac if it's just wet. The bigger issue is rolling resistance which is where off-road tyres fall down. Doesn't bother me, I like tyres that let me have a laff & jump off stuff, so I'm not about to fit narrow slicks…

Your theory is wrong for a start. When you brake your weight is moved forwards loading the front tyre even more than normal and un unloading the back hence its important to use more front brake than rear as you get far more grip with the front. Thats also why its easier to lock up the back wheel and much harder to lock the front.
However, in the wet its probably better to use a bit more rear brake as with the reduced friction between the tyre and the road its starting to get easier to lock the front. Rear wheel skids are much easier to control than front wheel ones.

Well, as I said it might have been part psychological and I might have been a bit nervous, maybe they were ok. The Razers have very widely spaced knobs which I thought might makes things sketchy on tarmac but in dry they've been really grippy and I've felt much more comfortable with them than the conventional semi-slick design where there's a scary transition from a fairly slick central tread to big blocks when turning. Since the knobs are so shallow perhaps there's more rubber in contact with the road as the tyre deforms at the point of contact.