I think there are three combat issues that often get conflated but are different. The first is grind. The second is combat pacing taking too long. The third is lack of threat.
The grind issue has to do with the combat reaching the point where the PC's victory has become a foregone conclusion, but there are still a couple of monsters that need mopping up and it seems like its taking a lot of effort to just take them down.
There are different ways of dealing with grind. One way is simply through encounter design. Avoid using soldiers, and avoid monster powers that cause Weaken or that take away turns. Those all contribute to grind.
The next way of dealing with grind is through house rules. Bonus damage, halving HP and the like. Those are good as well. I like bonus damage myself because it also addresses the issue of threat which I will get to in a moment. And of course, don't be afraid to have monsters surrender or flee instead of fighting to the death.
The next issue is combat pacing. To be brutally honest, I believe this is a player issue more than a rules issue. A few years ago my group participated in the WotC Ultimate Dungeon Delve which is a timed event. During our training sessions prior to that event we really deconstructed why combat seemed to take a long time and we found that it wasn't the rules. It was us. We took far too long on our turns by not paying attention, by making separate multiple die rolls, and by not really knowing how our PCs worked.
We made an effort to know and understand how our powers worked and to pay attention. When our turn came up in combat, we were already prepared to act. And we rolled attack and damage dice together. We went from about 5 minutes a turn per player, to less than 30 seconds a turn per player.
The nice thing about this process is now that we know how to play fast we can turn it off an on as we need to in our home game without feeling like we are in full delve mode. In my last two hour 4e session, I ran 4 full set piece battles and we still spent a good 30-40 minutes solely on NPC interaction and RP. Most our combat time was spent taking up setting up the battle scene and grid (one of my few issues with 4e, but I digress). We never felt combat took too long.
The third issue that I see come up in regards to 4e combat is perceived lack of threat to the PCs. There are two ways to address this. First any pre-MM3 monster simply doesn't do enough damage. This was rectified by WotC in MM3 and later, but a good rule of thumb is to give them +5 bonus damage per attack per tier of the monster. The second way to address this is for the DM to not hold back.
And by not hold back, I mean you should still strive to make encounters that fit the narrative and our balanced within the parameters of your story. But once the encounter is designed and the players get involved, don't pull punches. In prior editions, the DM really had to always play with one hand tied behind their back. But in 4e you generally don't have to worry about that. I go for the throat with my monsters in 4e. They will target the weak PCs, the controllers and the strikers or leaders first, then the defenders last. Its up to the players to use their marks, or other powers to stop me with clever tactics. If they don't, they die, because the monsters will fight as a team and will take advantage of conditions that grant them bonus damage and such.
