Anime Sometimes Less is More
Dragonball Z is that kind of show that is both insanely popular but also forever copied and endlessly parodied. It's strength always lay with the fun characters and armageddon level property damage. But when the average one-on-one battle takes 5 episodes (Frieza threw off the curve), you are bound to get some complaints of a Franchise Zombie drooling over selling DVD's where nothing happens.
Dragonball Kai manages to take everything great about Dragonball Z and take out the less impressive stuff. Fighters don't pause to stare at each other, there is little "cut to: shocked onlookers" and there aren't any Filler stories to inflate the episode count.
This all results in a story that is familiar yet comes to you in a new way. For instance, the Saiyan Saga features the deaths of many significant characters. The original show had their deaths take place over the course of about 6 episodes, each one highlighting their particular struggle and spending a lot of time with how they die. Dramatic stuff to be sure. But with Dragonball Kai those 6 episodes are trimmed to about 2 1/2, and what happens is that your friends are dying around you and there is nothing you can do about it. The nature of their deaths remained and aren't "glossed" over, but it becomes significantly more poignant by not drawing it out.
And a similar thing happens with the more drawn out one-on-one battles. These characters take a lot of punishment before they finally go down, mountains are destroyed before they bleed. But with the excess gone their near invulnerability is replaced by strategy (as the one tactic featured per episode now happens every few minutes) and when the bruises appear faster it highlights their power.
That's not to say everything is perfect, sometimes basic continuity suffers. Gohan goes from survival training to sparring with Piccolo with no introduction (The original had a poignant moment where Gohan almost made it home on his own but took responsibility to train with Piccolo). Nappa discarded his damaged armor with the others intimidated he was willing to fight without it. All of that was cut it all out, so Nappa is just suddenly almost naked.
All in all though, recommended for anyone who has wondered why Dragonball Z was one of the most popular animes ever.
Anime When the nostalgia goggles take over
So no doubt about it Dragon Ball Z is one of the definitive animes for 90's kids. Even if you didn't turn into an anime fan, if you grew up in the 90's you probably watched DRAGON BALL Z. One of the many iconic shows of the 90's but something fans and non-fans can agree on is the pacing is so bad. Both the fandom and hatedom have made jokes about how 10 episodes at a time would just be characters talking, so a recut of the series that cuts down padding and omits the filler should be perfect for everyone right? Wrong apparently. It seems that many viewers prefer the original Dragon Ball Z and a lot of that appears to have to do with the Nostalgia Goggles.
Music: Now whenever an English dub creates its own soundtrack there will be always be a Broken Base if the original score is better or the Dub Score. Of course for many fans who grew up on the 90's dub hearing the original Japanese soundtrack in Kai is probably off-putting. But even if you don't like the Japanese score is it really a reason to dismiss Kai?
The Voices: Another common criticism I heard is that the voices in Kai are awful. Um what? Most of the voices in Kai are the EXACT same voices as Dragon Ball Z, a few of them have changed such as Frieza. But these are for the better. Which do you prefer Frieza as an old woman doing her best gollum or a Frieza that sounds sinister (and British?!)? If you picked the former, the Nostalgia Goggles are on way too tight.
Censorship- Another complaint is about censorship, now if you're strictly comparing the Japanese versions I can see where this argument is valid, but for the English dub even the Bowderlized Nicktoons version is much less cnesored than Dragon Ball Z in the early days. None of that Another Dimension crap or HFIL. There is also an uncut dub version just like Dragon Ball Z has an uncut dub.
In short Dragon Ball Z Kai is MUCH MUCH better than Dragon Ball Z, and if you prefer DBZ its time to take off the nostalgia goggles.