Tire Recommendation

It's all about traction.

I think semi slick tires work well enough out here in the summer. I switch to some really aggressive mud tires in the winter months.

There aren't any hard or fast rules to tire selection and most of it is subjective. Things to take note of would be skidding, rear tire spin (tire spins and you don't go forward), and the dreaded front tire wash out (pretty much a guaranteed crash). A different tire may be able to help. There is only so much a tire can do in some cases. A 40% up hill with 3 inches of peanut butter like mud is likely to stop you dead in your tracks regardless of your tires. Picking the right tire for the right trail is something of a voodoo art.

There are some economic considerations in all this as well. Ideally, you would have this monster selection of tires and pick the right one for each trail. Unless your sponsored and someone else is picking up the tire bill, that is not going to happen. Additionally, high performance/high traction tires tend to wear out fast. They are usually made of softer rubber compounds that just don't last very long.

Generally speaking, big knobs with lots of spacing will clear mud really well and work in loose or muddy soil. The big knobs will flex around on hard packed surfaces and feel squirrelly. Logs and roots are hard and the big knobs will flex on those as well. Low tread and semi-slick designs will have less rolling resistance and handle well on hard pack and pavement. They don't do well in the mud and may have trouble in loose dirt. Then there are all sorts of things in between.

Brent

I am using Continental Vertical 2.3's on my full suspension and have been very impressed (Thanks Gabrielle). The Maxxis Ignitor is very similar. Both have pretty open tread patterns and good volume. Although as Brent said it is good to have options, winter and summer demand different rubber, as does racing vs. trail riding.     

My favorite all-around tire is the Kenda Nevegal.  It is a little slower than others on the hardpack but works great all year long.  They shed mud well also.  I use the 2.1 Dual Tread Compund (DTC).  They come in other sizes and compounds as well depending on you rriding needs.  The Nevegals have also won the Mountain Bike Action tire shootout now for the last few years.  If you look around you can find them on sale, otherwise they are a bit pricey.  I picked up a set at Fat Tire Farm with our PUMP club discount.

Fred