HQ
Hive Tyrant
In comparison to the 2nd Edition she appears fairly weak, most of her statistics have gone down, some of her abilities have disappeared. Do not be disheartened. She has weathered the storm admirably. Her WS, although lower, is still as potent. When attacking anybody with a lower weapon skill she needs a 3+ to hit, and it will never get worse than a 4+ to hit. Her shooting ability (BS) although lowered to match the rest of the army, was never that impressive. Although limited in weapon choices, both the Venom Cannon and the Barbed Strangler seem viable options. Her Strength and Toughness although they have stayed the same, make her a very tough opponent now. As every body now uses their Strength value in Assaults, her Strength & Toughness values are now very good. The number of wounds she has makes her now more potent than ever: no more one shot kills from an Assault Cannon. As each weapon can now only inflict one wound, at least four hits from a selection of weapons need to be successful. Even though her save is only 5+, it counts as an invulnerable save, which means there is always a chance to keep her alive.
Elite��
Lictor
Although you can only have three in a detachment, I think they have improved over the previous versions. Although the statistic's line has not improved greatly, I do not believe it has decreased badly either. The major change is in their infiltrating ability. Even though it is now dependent upon the mission, this combined with the ability to Deep Strike, allows you to use either in most missions. As you no longer have to deploy infiltrators out of sight, it is a lot easier to find a suitable place for these to go.
Tyranid Warriors
I do not believe the statistics for Tyranid Warriors have changed greatly. There are two major changes that concern Tyranid Warriors. The first I believe to be the lack of bio-morphs. These made my Tyranid Warriors more survivable under 2nd edition, although hideously expensive. The only option now is for fleshhooks, which will be useful. The second change is that the Bonesword is now a normal close combat weapon. This is a shame. This means Tyranids Warriors are no longer able to carve through power armour in the same way as before. The good side about 3rd Edition though, are the long distance weapons. No longer does the Venom Cannon encumber my Warriors. They can all fight in close combat with nearly the same ability.
Troops
Genestealer
The major change is the strength decrease. This makes it almost impossible for a Genestealer to kill vehicles, and harder to kill large creatures, and virtually impossible to kill vehicles from the front. The other statistic changes make it a little harder to receive the charge and still not kill everything, but otherwise they function with nearly the same effectiveness as before.
Termagant
Primarily used as cannon fodder in 2nd edition, they appear to be a vital part of your army for the 3rd Edition Not very much in the way of statistic changes for Termagants.
Ripper Swarm
These have undergone a major change since 2nd Edition. You can now take these as one of you troops' choices. This allows you to take many more of them than you ever would have before.
Hormagaunt
Hormagaunts have slowed down considerably in the new edition. Even though they can now Assault 12 inches, they are still moving at the same pace as the rest of the Swarm. They have also lost their second attack, unless you count the bonus everybody gets when you Assault. They have also had a price decrease and cost less than a Termagant.
Gargoyle
Gargoyles have had a dramatic reduction in their flying speed, although this is not as bad as it sounds. They can move 12 inches in the movement phase and 6 inches in the assault phase. This is fast movement under the new system where most can only move 6 inches. Their main weapon is now a Fleshborer, which in itself is a reasonable weapon.
Heavy Support
Carnifex
Although from first appearance he looks a lot weaker, the Carnifex is still a viable unit. His Toughness has decreased but as all weapons only cause one wound, he can still withstand the hail of fire coming his way. The Bio-plasma attack is now optional and counts as an assault weapon. This is a lot more useful than before.
Zoanthrope
The statistics on this creature have not changed greatly, but the ways it�� powers work have changed it for the better. The Warp Blast is now a shooting attack, instead of a Psychic attack. The Warp Blast has the best AP of the swarms' weapons. It also uses the 3 inch template, so should be able to destroy several Terminators, if it can hit. The Warp Field that used to surround the Zoanthrope has changed. It now gives the equivalent of a Terminator save.
Biovore
The only change is that you now have to guess the range of the spore mine. It will still scatter from you estimate, but by turn three you should be able to guess most ranges fairly accurately. The Spore mine has changed in power. It is unlikely to damage most vehicles with its explosion, but with it's five inch blast are should hurt most troops.