Doesn't have any buttons to make you play better 
But you will have awesome tone 
It won't keep your coffee warm like a tube amp ...
If you use stompboxes with unique sounds in addition to your amplifier, you many need to keep using those stompboxes. It cannot replace literally every pedal ever made.
Sadly It won't hum, scratch and get you into electric shock like your vintage tube amp : it's safe , noiseless & hyper clean.
More seriously it lacks a spring verb , but it will eventually come with a firmware update.
It can't do:
-USB audio
-synths
-step filter FX
-realistic talkbox
-vocoder
-dual amps
-pitch to MIDI
There are also some control limitations that don't exist in other units, but I don't know if you want to get to that level of detail. It's still my favorite gear purchase of all time.
You have to decide between a powered or non powered toaster.
No spring reverb.
And also it is called so: it can not toast Tour bread!
No spring reverb.
And also it is called so: it can not toast Tour bread!
... so far... 
Hey guys,
I'm so freaking close to buy a Kemper.
So I thought before I buy this awesome-do-it-all of an Amp (or Eierlegendewollmilchsau as we say in Germany) I wanted to ask you what it can't do.
Hope I don't have to but it's the Internet so keep it Guitar-related.
I know it sadly can't fly and befriend a unicorn...
It won't emulate how the knobs on real amp work. Hence, some people keep 100s of profiles of the same amp.
Contrary to what some people may think, it cannot make toast.
It will defo not let you sleep better 
It can't profile time-dependent signals. That means that natural room reverberations are not captured in the profiles themselves (though the unit has built in reverb EFFECTS.). It also cannot capture time based effects inside or in front of the amp being profiled (chorus, tremolo, flanger etc). Again, it has those effects built in.
There are also non- serious answers allowed?
OK, here is semi- serious.
It won't make you re-tweak your rigs after a FW update.
You don't get the ability of a fuzz emulation to interact with your guitar pickups..... A vintage fuzz is a 'flawed' design in that it is a low input impedance - that's one of the reasons the volume knob on your guitar acts the way it does with a vintage fuzz. Of course, you can use your actual vintage fuzz in front of the Kemper itseld and I can confirm that that works just fine 
You can't do the 'two Amps at once thing'. Hasn't been an issue for me at all but you're asking
You can't put reverb or delay BEFORE the amp. It's been requested that this is changed and hopefully it will be.... Along with spring reverb
enjoy it when you get it - it's a fantastic box of joy
No Delay before the Preamp, no Reverse Delay and as mentioned no Spring Reverb (though I use a simulated one in my Delay section that's pretty cool, was posted free on the rig exchange).
