They tell me I'm the resident 'Puppy' expert round here (groan... ) Be that as it may, when I started off in Linux over 2 years ago, I also began with the 'buntus.....because that was what everybody and his dog (sorry; couldn't resist that..!) recommended.
The big drawback with Ubuntu itself is the Unity desktop. It's fine to use, so long as you have 3D acceleration capability in your graphics hardware (ideally a discrete card), along with plenty of dedicated graphics vRAM. It does need a fairly capable machine; and, although I've said it before, I'm going to repeat myself yet again. Canonical, in the person of its leader, Mark Shuttleworth, are starting to branch away from the original Linux 'ethos' of trying to keep older, weaker hardware alive & productive. Mr. Shuttleworth has decided he wants to be the Bill Gates of the Linux world, so Canonical are steadily dropping support for older hardware in favour of newer, more powerful stuff. It's becoming fairly obvious that Mark is trying to compete directly with Microsoft.
This is where the 'flavours' come into the equation.....particularly Xubuntu & Lubuntu. Kubuntu is a bit of a 'heavyweight', using as it does the KDE 'Plasma' desktop, which requires about a million and one dependencies before it will condescend to run..! It's great fun to play around with and customise, but, like Ubuntu's 'Unity', it needs plenty of graphical 'oomph'.
Xubuntu uses the XFCE desktop, which to my mind is one of the most highly configurable lightweight DE's (Desktop Environment.....more 'Linux-speak', I'm afraid; you'll pick it up in time!) available.
Lubuntu, if I remember correctly, uses the LXDE desktop, employing the 'OpenBox' window manger. It's a fine combination if you're the sort of person who is more concerned with actually doing stuff on your machine, rather than with how pretty it looks.....although it's just as configurable as the rest of them, if you take the time to dig around in the settings. I've had mine looking just as good as Windows 7 at one point. I ought to have taken a screenshot of my efforts at the time; it really would have given you some inspiration in that respect..!
The thing with all of them is that they all employ the same core system files (the same 'engine under the hood', if you like.) This is why they're all able to access the same sets of software repositories. The different 'flavours' are developed by different teams, who place different priorities on what they see as important.
Re: your example with Libre Office. The reason why Lubuntu comes with Abiword as standard is that it is produced as a complete system using a set of more 'lightweight' applications, in keeping with its more 'lightweight' nature. You can get LibreOffice, as the others have stated, from the repos; you can also, if you want to keep up with the very latest version of LibreOffice all the time, download and install the .deb file directly from the LibreOffice website. It's a reliable, clean link; I've used it myself in the past with no problems whatsoever. It's not, however, recommended practice.....but it will work. You take the risk on yourself.
I only say that last part because one complaint you often hear from 'buntu users is that the versions of software in the repositories are very often a couple of versions behind whatever the current, cutting-edge release happens to be.....and they don't tend to get upgraded until the next 'buntu release; a minimum of 2 years for LTS releases.
It's the same with 'Puppy'; because it's a very lightweight distro, it comes with very lightweight apps.....but it's surprisingly full-featured for its diminutive size..!
Distros:- Multiple 'Puppies'..... and Anti-X 16.1
My Puppy BLOG ~~~ My Puppy PACKAGES
Compaq Presario SR1916UK; Athlon64 X2 3800+, 3 GB RAM, WD 500GB Caviar 'Blue', 32GB Kingspec PATA SSD, 3 TB Seagate 'Expansion' external HDD, ATI Radeon Xpress 200 graphics, Dell 15.1" pNp monitor (1024 x 768), TP-Link PCI-e USB 3.0 card, Logitech c920 HD Pro webcam, self-powered 7-port USB 2.0 hub
Dell Inspiron 1100; 2.6 GHz 400FSB P4, 1.5 GB RAM, 64GB KingSpec IDE SSD, Intel 'Extreme' graphics, 1 TB Seagate 'Expansion' external HDD, M$ HD-3000 'Lifecam'.