The demo is pretty cool. Hopefully THQ got the message, because the first two games had some major flaws.
Here are some new features, taken from the wiki. I highlighted some of my favorites
New THQ servers for online play
Pride Mode, with Pride rules, ring, and fighters. Commentary will be provided by Bas Rutten and Stephen Quadros. Lenne Hardt will announce the fighters in Pride.
A brand new submission system.
Mo-capping used for fighters giving more realistic movement and striking.(This doesn't apply for all fighters. Some will still use archetypes.)
New Stats: Footwork, Ground Grapple Top and Bottom for offense and defense.
Two gameplay control options: traditional and simplified. (Pro and Amateur controls)
Leg Kick TKOs, including leg breaks from checked kicks.
New submission moves including the standing rear naked choke, standing guillotine, and the flying scissor heel hook.
New KO animations.
Damage recovery between rounds.
Cuts affect stamina and energy recovery rate.
Redesigned striking game emphasizing jabs, quick strikes, and combinations.
New Stamina system. Your stamina can only come back up so far until you are fully rested.
Stamina depletion will also effect fighter's stats.
New clinch controls. You can close the distance to keep from getting knee'd to the head & punching your way out. (This also helps in other areas)
New transition in the clinch to back control.
Whip knees can be done to the body & can be blocked.
Stat decay removed in Career Mode.
Roster fighters from the UFC and Pride can now be taken through career mode.
New ground positions with the cage.
Using the cage to set-up submissions & "wall walking" back to a standing position.
New options, such as Stamina Simulation mode, Competition Spec mode (Removes all random elements such as flash KOs and doctor stoppages), and Stat Equalizer (Equalizes all stats to 80 for both fighters).
Reach is now a big part in striking.
Fighters are now able to sway on the ground to avoid strikes.
A new feint system.
Takedown-intercepting knees have been added.
"Finish the Fight" allows you to continue striking until the ref pulls you off.
Sweeps are back including new ones.
Elbow spamming has been fixed. Only certain fighters have elbows in clinch range.
Ref resetting has been fixed. You can only pull someone off the cage by a takedown/slam.
No more sub-cancelling.
Pride Grand Prix have been included, with the ability to have fights in the same night with damage moving onto the next fight.