WIN: If Microsoft goes the incremental route with the same Xbox OS a-la Apple's iOS evolution, they keep middleware compatibility with larger graphical budgets for teams to play with... EVERYONE likes this and every dev team that has shipped an Xbox 360 game hits the ground running immediately -- possibly opening up the doors to dual Xbox 360 and Xbox 720 releases of their games (one with lower quality textures and rougher geometric assets and the other with higher-end business) -- I am sweeping my hands across a fairly complex problem here, but you get the point. With compatibility intact, there are interesting things that can happen here that we've seen work on the iOS ecosystem. (maybe not for Triple-A devs, but for budget minded games, this could work wonderfully especially as digital-downloads and indie devs grow).
LOSS: Sony has to make a clean cut and scuttle the last 8 years moving onto an x86 design, new OS, new middleware all new QA cycles and retraining all the PS3 devs.
WIN: Sony now matches the same arch as the Xbox-next (same CPU optimizations apply and same GPU family) and we can stop getting these damn port-style games that never take advantage of the extra Blu-ray room or cell processing power.
LOSS: No revolutionary boost in performance if the hardware rumors are true. I have no doubt the bump in specialized/tuned hardware will give us real-time versions of the Unreal Engine 4 demos (of that smoking guy that beats up mechs) BUT, at 4k resolution and 120hz for 3D? Not for every game. I imagine we will see something akin to what we have now with most games running at 720p with the few rare tuned ones actually running at 1080p resolutions. I would guess next-gen will offer 4k resolution on some titles and 3D on others while running at 1080p or some such combination. Obviously if it is a simpler game like a Wave Racer, we'll get both, but for Gears of War 15 and Mass Effect 32, I doubt it.
WIN: Faster game dev cycles means we get some more impressive titles on the improved hardware/experience sooner. Don't have to wait 2 years post-launch for the first real good game.
LOSS: Next-generation consoles are going to be engineered to be all-in-one entertainment hubs... TV cable tuning, DVR, streaming, apps, social, games, mobile-tie-in, etc. etc... this will make jumping between consoles harder as you stake your claim in the Microsoft or Sony (or possibly Nintendo -- yet to be seen if they can pull off a Network) camp and build up your life/existence within their walled garden. This generation it was nothing more than rep points, next-generation (especially Microsoft) will be throwing every Facebook-esque psychologically engineered trick in the book to keep you ON their platform, consuming content through their channel with your credit card en-tow, sharing your pictures with friends and building your character's rep from game to game to game, movie to movie and app to app.
I sincerely doubt we (the harder-core folks) will all have all 3 next-gen systems this next time around and will most likely have 1 (probably Xbox-next) as the cost of flipping between them will be bigger than just powering on Console X to play game Y for a few hours.
LOSS x2: Double the resolution on the Kinect-next, rumored eye-tracking, multi-person tracking and more accurate voice controls? I can't even imagine what is in store (e.g. "You don't seem to be enjoying this episode of Lost, would you like to skip to the next episode or can we recommend Battlestar Galactica?" -- say "Start Mass Effect, invite Scotty32, Soldier load-out" -- experience {episode of Law and Order, McDonald's Billboard in background... notification popup...} "Just emailed you a buy-1-get-1-free BigMac coupon for McDonalds! The nearest one is 2 blocks from you... yummy! Say 'Pause' to pause what you are watching and go grab lunch!")
You get the idea...
[1] http://en.wikipedia.org/wiki/Wii#Technical_specifications