Random Charge Length -
Both the best thing and worst thing for new assaults. Essentially, every single assault in open-terrain is comparable to an old assault through difficult terrain. I.e. anything further than 4" and you're approaching double digits in terms of failing percentages. Chuck in real terrain and that drops to 3". Yes the most common number is going to be 7" but there's still quite a lot of fail possibilities there and overwatch can quite likely add an extra inch or two to that distance. This post here on the forums has some nice numbers accumulated by multiple posters looking at charge range probabilities. These numbers emphasis assaulting anything higher than 6" is really risky (particularly since even if you fail, you take Overwatch shots).
This is where RCL hurts - assault lengths are essentially lower than before and are less reliable. The other side of the sword though is when you do get those 7"+ charges and particularly the 10"+ charges (i.e. nearly double the previous max). Having assault units charge out of what could be no where is very scary for shooting units. This is particularly true in a couple specific situations. One, where the unit charging is very durable - i.e. overwatch firepower is unconsequential such as in a deathstar like unit or a walker. Two, against units which have unconsequential or null overwatch fire - i.e. small squads, blast weapon heavy squads, vehicles, etc. Obviously being able to continually attempt charges and shrug off the extra firepower normally associated with these risky manuevers can make those uncommon charge lengths more common and thus have a bigger impact upon the game.
An addition to the above, Fleet is huge. If you don't have Fleet, cry. Let's put it this way. Fleet through cover, is better than a normal charge through clear terrain. It's that good.
Pulling Punches -
As in 5th edition, it's important to stay in combat for two combat phases when you initiate a charge. This means you're engaged in your opponent's next shooting phase but pop out before your next movement phase. This limits a lot of the advantages shooting has (the unit cannot be shot) and allows the assault unit to engage the opponent again the next turn without wasting time. Not always possible but it's always better to start a combat and stay in it for an even number of combat phases so you don't pop out during your opponent's shooting phase.
So how has this changed really for 6th edition? The linked post explains a lot of the basic concepts and two things help change this up. Challenges & disordered charges. Disordered charges allow you to over engage chaff units a little bit better by ensuring less attacks are being directed at the opponent. This is great for staying in combat when the unit in question is likely to destroy their intended target regardless and even if it fails, you can kill two squads instead of one. Remember though, multi-charges are much harder to pull off. Disordered charges also reduce the effectiveness of such squads when you want it (i.e. against a parking lot, two tougher units clumped together, etc.).
Challenges also allow you to take attacks away from a squad and drop them all on a single enemy model. There's a couple of issues with this though - challenges can be refused and even if accepted, it likely loses the squad some leadership benefits. If not accepted though, you ensure the squad keeps their higher leadership and forces the opponent to take such a decision with less information (i.e. before attacks are made). Bit of tit for tat here but a useful tool up your sleeve regardless.
Initiative Based Pile-ins
Pile-ins are done very differently compared to 5th edition. Rather than all being done as combat engages or at the end if combat continues, pile-ins are now 3" and done at each initiative step. The reduced pile-in move makes strung-out combats more likely to finish if both squads smack each other pretty well and limits the distance pulled for engaged units. The initiative based steps generally has little impact if squads fully or nearly fully engage each other on the initial charge but for particularlry strung out squads, an odd parody exists. If you have a higher initiative and are able to do enough damage, you can effectively remove the opponent's ability to pile-in to make attacks (but still stay engaged in combat). If this does not happen though, the lower initiative models have more options in being able to move closer to the opponent (who have also had their 3" move) and thus get more attacks in. Lots of way to use this for both sides of the coin in terms of unit-wide initiative but it generally has more impact on individual models with different inititiatve values compared to the squad (i.e. generally lower liker a Power Fist) as this gives them more information of which to act upon.
Conclusion
The big fish here is RCL and how it impacts the use of general run of the mill assault units, how important Fleet is for making those units work and how potentially scary Overwatch immune/resistant units can be with much higher charge range potential. Beyond this shooting is still the big fish and RCL generally doesn't impact this at all. Staying in combat is a bit easier and combat is a lot deadlier against vehicles but if you can't stay in combat, shooting is going to smack you around. Iroincallylower initiatives can be more effective when units arestrung out and engaging each other as you end up with a greater aggregrate distance gained and have more options in getting your models engaged.
