At entry level it's all pretty much of a muchness. However, once you get into specialist XC race bikes the weight, geometry and equipment will make the bikes far more "task specific", so you'll find 9kg carbon bikes with 80-100mm forks, carbon wheels, skinny (for knobblies) tyres, 140mm brake rotors, narrow bars and a general "arse up head down" position. Front head angles of 72 degrees mean the bikes are awesome for climbing and pedalling, but will be twitchy on descents and not designed to jump or drop. For what it's worth, a lot of serious XC bikes are 29ers. Big wheels able to roll over rough ish terrain with composure, but not necessarily all that good on tight twisty technical stuff.
UK trail riding is pretty unique, so much so that some manufacturers make specific configurations solely aimed at the UK market. We differ because a lot of Euro or US riding tends to be either pure XC, gravity based Alpine riding, or Enduro style stuff, where races are timed on the descent sections, and the climbs are non-competitive. We are therefore pretty unique in wanting bikes that will climb as well as they descend.
There's a specific genre of trail hardtail bike for example, known as the LT (long travel) hardtail that has an astonishing following. These are often niche or boutique brands, often with a whiff of mystique about them, and are almost always custom built. They range from the cheap and cheerful steel frames from the likes of On One, (456, Inbred) through new kids on the block Ragley (Blue Pig, Piglet, Marley) through seriously tough stuff from Dialled Bikes, NS, to exotic steel alloys Privee, Sanderson, Cotic and so on. They tend to be run with 120, 130 or 140mm forks, and some are capable of taking eyewatering abuse. These are the "trail hardtails" and have compact playful geometry, with short effective top tubes, snappy little chainstays and slack head angles (my Cotic Soul has a 68 degree head angle with a 120mm fork). It rides perfectly happily on UK red routes, demanding finesse and careful line choice, but rewarding with a sweet, fun compliant ride. Don't think that it can't be ragged about though, it's an absolute hoot to ride fast and hard on the sort of rocky nonsense that passes for bridleways here in the South Pennines.
Full suss trail bikes are best described as burlier versions of XC bikes, but without getting so porky that they become difficult to ride uphill (or all day) . They are most commonly fitted with forks between 120 and 150mm travel. Rear suspension tends to be between 120 and 140mm. Forks will often be lightweight, and in fact general componentry will be aimed at lighter weight whilst retaining sufficient robust builds to withstand jumps, drops, rock gardens and so on, so they'll often have 2.35-2.5 inch tyres on strong wide rims. Bikes in this bracket will tend to weigh around the 12-14kg mark, so they won't race well! We start seeing dropper posts, double and single chainrings in this category. There are still a lot of 26 wheels in this genre, and the latest breakthrough would appear to be 650b. Typical Trail bikes would be Orange Fives, Santa Cruz Blur LT, Yeti 575, Specialised Pitch, Giant Trance, Trek Fuel.
Enduro racing is proving to be very popular, and again, the market is starting to pick up around even burlier builds, with 160-180mm forks with 36mm stanchions for serious burliness, and 160mm rear travel. Heavier, stiffer frames with slack (66 degree typically) head angles, reinforced sidewall tyres, plenty of 1x10 transmission and builds of 30lb plus (14-15 kg not untypical).
I ride with Shimano XT Trail pedals on all my off road bikes. Simple SPD system with a caged surround for dabbing purposes (gives a good target for clipping back in)
