Chaos Marines - too much like space marines to really be of much difference on the table. They share the same strengths - durable, good shooting, good assault, unique units.
Dark Eldar - Actually, oddly enough, they are very much like Bugs in my opinion. They can be very fast with many small arms firing each turn. Lightly armoured with a few exceptions, and can pack a solid punch with the few heavy weapons they'll fire each turn.
Eldar - Very much like the Tau in many respects. Very shooty if designed to be so, and some of the most useful vehicles in the game. Differences being that the Eldar actually have an assault element, though many players seem to disregard this major point.
IG - All the weaknesses of the Bugs with the shooty ability of the Tau. A horde army that can sure deliver the pain but just can't manage against a fast army.
Necrons - Marines with Feel No Pain and very limited options. A very nice convertaholic's army, but will get very dull after playing Marines.
Orks - Bugs with rides. A largely combat oriented force which relies upon numbers and speed to close with the enemy in order to dish out the pain and take control of the field. Fun army, but not too different from the Bugs.
Witch Hunters - Imagine a cross between the Marines, IG, and Eldar and you get the Ordo Hereticus. Special units all over the place with many many interesting rules to confuse yourself with. It can be a blast, but is considered a pretty strong list when used with competition in mind. Generally, though, they'll play much like Marines and Marine Scouts, but with a few extra options.
Daemonhunters - My pic of the list. If you have been successful with your current armies, then you have mastered the Balanced game, the Assault game, and the Firepower game. Now you need to test your mettle with the uber-elite and see what you can make of the specialised Ordo Malleus units. Grey Knights, both PA and TDA, are expensive but very powerful. Their true weakness is in their inability to field more than a handful at a time and make that mere handful work the magic. The Inquisitors of the Ordo Malleus are the most difficult to perfect. Their options seem to be designed to keep you guessing. But, overall, these guys are great at taking out multi-wound and psyker heavy opponents, and can really surprise an opponent throughout the game with their deepstriking PA and TDA GKs and mobile firebase ISTs and Inquisitors.
Hope this at least gave you an idea or two.
