New OpenGL (with known features)
New D3D8
Client is compatible with current servers (aside the ugold/226 server problem which was caused by legend and can't be fixed here, because UGold server is the problem, not the client)
227 server is compatible with older clients (224,225 and 226 OR 224,225 and UGold) when anticheat is disabled (no good idea to do so

, but admins choice)
What does not work:
Current anticheat mods, anticheat protections and similar tools. With the new anticheat protection those should be obsolete anyway.
I presume something like the last released version of Hackbusta will not work in 227 then. But last released UProtect will work fine like you mentioned?
If wanting backwards compatibility older protections won't be obselete, I know of a lot of people who use ugold, and it would be crazy not to include them if running a server I believe. I can get some people to install 227, but it's impossible to reach everyone.
I wonder what improvements D3D8 will bring. Many (including myself) prefer the older engine found pre 226/ugold (same as ut?) I use glidewrapper 225f and an important thing for me is vsync disabled to improve mouse movement/aim even though it brings tearing on flickering lights e.g. deck16 flak/biosuit. In gold the vsync ini setting under d3d of true or false is ignored and always stays set to true in game from what I've seen, setting false in graphics driver options appears to disable the vsync for ugold. Will 227 fix this ugold bug? d3d in 225f does not work correctly on modern graphics cards, at least not on the last 3 generation nvidia cards I have had (5700LE/7800GS/8800GTX) - the game world is rendered, but the menu system (and hud too I think) does not appear (you still hear the menu sound when pressing ESC though). Therefore opengl/glidewrapper are the only options in 225. I can't remember if it happens in 224 too but it probably is the same.
I'm very curious what this mousefix for mice does, I've had many problems in the past in xp with my mx518 and tested all kinds of variations of settings/setups to try and emulate the great setup I had in win98. Turning off enhance pointer in xp mouse setting helps and setting mice to lower dpi (plus not using logitech driver can help I find), and for me vsync disabled. I hate any smoothing options because I feel they ruin the sharp reactions of mouse movement. I recently bought a razer deathadder but having trouble getting it detected/set up. My mx518 is dying (stutters/locksup/loses and brings back power randomly, wont keep to dpi setting), currently using my old microsoft optical mouse lol
I actually have old parts I've used for testing unreal on an older pc again (since I threw my old pentium2 pc away in 2004), I bought stuff from ebay including an old amd 450mhz, pentium 3 600mhz, voodoo (3 I think/forgot) card, geforce2mx, the motherboards and ram came with some of those. I didn't have to spend much money lol
I have the high speed cpu bug with vsync disabled in 225f glidewrapper and ugold d3d where offline speed is too fast to play normally but with vsync enabled mouse aim is terrible for me although game speed is fixed. Online with vsync disabled fps is fine and do not get the high speed unpredictable performance experienced offline.