Don't half-ass the roles of R2D2 and C3PO. Keep them true to themselves.
This means: no giving them stuff to do for the sake of giving them stuff to do, and no scenes with only those two that don't move the plot along or do *something* to develop their characters.
And if nothing comes to you, don't shoehorn it. So: no crazy stunts for R2 other than hacking computer systems and doing awesome ship maintenance. No killer C3PO or anything other than making him annoyingly prissy and a sometimes-useful translator.
Don't screw up your villains, and try to make them as novel and unpredictable as possible.
This means: no maniacal, single-minded Eric Banas or freakish but shallow Cumberbatches (...or Montalbans, for that matter). No master manipulators a la Palpatine and no lovesick but powerful shmucks, avaricious useful idiots, or disposable sidekicks for them to target or recruit. No extragalactic villains unless they are anatomically the same (in other words, not 'outside' of the Force like the Yuuzhan Vong).
No Galactic Senate or boring, pointless, or nonsensical politics in general. If there is politics in the film, the issues being hashed out should be real and central to the plot and not contrived bullshit. For example, no squabbles over Jedi jurisdiction or any shit like that.
No Naboo, no Wookiees except Chewie, no ewoks, no Yoda-race, no Gungans.
No cameos by any prequel trilogy character - especially not Anakin. Maybe I'll allow Yoda, but only in a voiceover flashback-type thing, and not onscreen. No Obi-Wan or Qui-Gon giving Luke advice though.
No Coruscant BS action nonsense a la AOTC. This is right up Abrams' alley and I'm scared for this. In fact, no Jedi action nonsense in general. Again, I have very little hope for this.
No 'new Han Solo', 'new Luke', 'new Leia' or anything like that. If the characters are to be phased out or killed (which I'm not automatically against), don't just lazily try to force them into younger bodies. In other words, make sure the new guys all have their own voices and don't necessarily have the exact same roles as those they're replacing.
No time travel.
There's more, I'm sure, but that's just off the top of my head.