This year's UK GT will be very interesting indeed.
Orks look intimidating but the changes to blasts (particularly for big blasts) mean their numbers won't last all that long. Also the boys really won't like it when combats go badly.
I think Tau will remain much as they are. A good tactically challenging army that'll rely on killing opponents and hoping nothing's left to contest objectives. Auto-obscured tanks look sweet but on the downside the new vehicle damage table and shooting really reduces their old flexibility. The new missions do not seem to favour Tau if they go second. Outflanking really upsets them. Kroot are much better with the new rules. (Outflank and go to ground make them so much more versatile.)
Marines both regular and chaos I think will keep their builds pretty much the same. Blood angels could become really annoying though. (I don't like the sound of 50 odd assault marines plus death company. Doesn't Dante look sooo much better with the new rules though.
Nids I think will change a lot. Without number has become very attractive and units like warriors are much more durable with the abundance of cover saves. Run means the slower combat stuff can now close range quicker and I'm loving my blast weapons now. Very little if anything got worse for them. (Lurk and go to ground for a 5+ cover save in the open.
Necrons I think may have gained a tank that is really now almost impossible to destroy but the new combat modifiers to moral mean I think they're going to be sorely pressed when push comes to shove. Lose by 3-4 wounds and they're likely to break and get run down. No we'll be back for them!
Eldar are still going to be horrible but they were anyway. Not as easy for them to win though. Objectives will make it a lot harder to win missions with tri-holofields and the like.
Dark eldar might actually become a lot more nasty. If they get first turn everything they can place goes on the board ready to annihilate the foe. If they go second everything goes in reserve. (Except maybe for a unit like grotesques and a character with webway portal.) I think they'll be one to watch.
The inquisition I don't see working much differently. Sisters will still be a good army and grey knights still very tricky.
Daemons were designed for 5th but from the few games I've seen they'll need a very good commander if they're to win.
These are my thoughts for the the GT.