Here are spoiler I got while I was browsing for it.
This is the first in a 6 episode arc that will lead to a confrontation at Nationals. Sue (after losing Regionals) is determined to make sure that New Directions loses at Nationals. To do this, she begins to enlist what producers have currently called the Legion of Doom in order to give Vocal Adrenaline the edge at Nationals. So far, confirmed members include Dustin Goolsby (Cheyenne Jackson), Terri Delmonico (Jessalyn Gilsig), and Sandy Ryerson (Stephen Tobolowsky).
April Rhodes (Kristin Chenoweth) and Sunshine Corazon (Charice) will return during this 6 episode arc and Holly Holliday (Gwyneth Paltrow) will be making her third appearance during this arc.
As seen on the promo, Will and Holly are taking their relationship to the next level.
Jessalyn Gilsig (Terri) was on set filming scenes. Cheyenne, Gwyneth, Stephen and Charice are also back. Stephen (Sandy) confirmed that he recorded and filmed the song "A Night In Bangkok".
Chord Overstreet filmed a scene and then went to record a song.
Amber Riley (Mercedes) tweeted that she has recorded a song. She also said she is filming a "very big scene".
Various SYTYCD members that were part of Aural Intensity have been tweeting about filming a Glee scene with a song. With work on Original Song completed, its assumed the students transferred to Carmel High and Vocal Adrenaline as part of Sue's plot.
Adam Anders confirmed that Lea would perform 'How' by Lisa Loeb.
Harry Shum Jr (Mike Chang) tweeted that he will do a solo.
Part of the script was revealed which said the following:
* Sunshine wants to sing at benefit.
* ? and Kurt in upper auditorium.
Santana gets slushied by Karofsky in the hallway while she is talking to Brittany.
Becky and Jacob will also be back, but what are they up to?
Sandy attempts to sing "One Night in Bangkok" but is booed off the stage. Sandy returns to McKinley to wage war on Will and the Glee Club.
Each member of the Legion Of Doom uses a codename:
* Sandy Ryerson as "Pink Dagger"
* Dustin Goolsby as "Seargent Handsome"
* Terri Delmonico as "Honey Badger"
* Sue Sylvester as "Zod". Rank of General to match General Zod of Superman fame.
Cory Monteith recently said in an interview "..A Night of Neglect,' which is an episode focusing on artists that may have been neglected or underappreciated over the years." It is similar to Season One's Episode "Bad Reputation".
Something goes on between Artie, Mike, Tina and Brittany.
According to the promo, something happens to Mercedes that makes her more of a diva than she already is.
Each member of the Legion of Doom has a specific mission that they will carry out over the course of the back 6 episodes. Sunshine is not a member of the Legion of Doom like some have assumed.
The last 6 episodes will revolve around New Directions battling Vocal Adrenaline and making the trip to Nationals. Ryan confirmed that Charice has already been shooting scenes for this final arc of the season. Kurt returns to New Directions.
The Warblers sing Somewhere Only We Know to Kurt.
A Puck/Rachel scene was filmed.
Finn and Rachel do not get back together this episode.
Sue gives Terri the mission 'Destroy The Glee Club'. She begins her mission next episode and thus isn't part of this episode much. It is suggested that Terri will meddle with Emma and Will and that it might backfire on her.
So far there isn't much between Santana and Brittany, but Santana isn't going down without a fight. 
Kurt returning to New Directions sparks a pretty important conversation between Kurt and Blaine. This is not a 'fight' but a very important 'conversation'. Apparently it doesn't involve kissing or happy times. Kristin emphasises the words 'conversation' and 'fight' possibly hinting at something.
The Warblers will be singing "Somewhere Only We Know" by Keane. This should be the "goodbye song".
Brittany and Artie are still together.
A VERY unlikely "romantic" pairing is going to take shape.
The Warblers Glee album has revealed 3 new songs by them. Each is grouped by episode, thus song 11 must be Kurt's 'Goodbye' song.
This episode is not part of May Sweeps because the Sweep begins on April 28th which is not during the timeframe in which this episode is in.
Despite popular rumor, Sam and Rachel are not becoming a couple anytime soon.
New Directions will be singing an Adele Song.
Songs on 'A Night of Neglect' :
* How by Lisa Loeb. Sung by Rachel.
* One Night In Bangkok by Murray Head. Sung by Sandy Ryerson.
* All By Myself by Eric Carmen. Sung by Sunshine.
* Bubble Toes by Jack Johnson. Sung by Quinn, danced to by Mike.
* Ain't No Way by Aretha Franklin. Sung by Mercedes.
* I Follow Rivers by Lykke Li. Sung by Tina.
* Somewhere Only We Know by Keane. Sung by Blaine and Dalton Academy Warblers
* TBA by Adele. Sung by TBA.
Guest stars on 'A Night of Neglect' :
* Cheyenne Jackson as Dustin Goolsby
* Charice as Sunshine Corazon
* Gwyneth Paltrow as Holly Holiday
Here is the trailer 'A Night of Neglect' :