I've recently got a warranty replacement frame (Whyte G160RS) and on open trails/bike parks where speed is easy to come by it absolutely rips but I find on my usual riding of tight and techy steep downhill trails its a real handful.
Other than having slightly more travel than the frame it replaced, the main difference is it is much longer with a top tube length of 636mm and reach of 479mm on a medium frame. I've changed to a 35mm stem down from a 40mm so don't want to shorten the stem anymore.
Has anyone else found that it is just a case of adjusting your riding style to suit a longer bike or is this an inevitable weak point of progressive "long, low and slack" geometry?
Everything about long, low and slack is for the benefit of high speed stability. Practice will certainly help you maneuver that boat in time, but it'll never be able to negotiate a tight turn like a shorter bike could. The opposite is true when it comes to high speed stability.
For this reason, many manufacturers offer both trail and enduro models.
That's a normal sized bike for me, its just getting used to the bike. Its not going to be nimble like a XC bike and you might have to man handle it a bit or slide it to make it do what you want.
I ride a bike with 510mm reach, and a wheelbase of 1300mm I also like steep tight tracks, and find that the extra stability and balance this gives me over my old, shorter bike increased my speed and confidence in the tight corners and steep drops.
You do have to adapt though, I seem to ride alot more over the front wheel than i used to.
I've got a mojo geometron and have no problems at all on tight techy stuff. In fact I would go as far as to say I can be much more aggressive and turn tighter with it. The key for me is getting right over the front which seems to be my natural style anyway and probably why i used to find shorter bikes tucking and washing out. Try really driving the front and just let the back follow.