Xonotic is a free, opensource, cross-platform arena shooter with UT- like alternative fire modes, weapon combos and fast movement. The simplicity of item respawn times makes Xonotic a good gateway for learning decision making and resource control in arena FPS.
Pro Interesting weapon design (for an arena shooter)
All arena shooter basically use Quake or UT weapon system. However, Xonotic has an interesting variety of weapons (larger selection than most) with elements from both UT and Quake but added some of their innovative ideas. Some of the weapons have a bit of overlap and felt a bit forced, but other weapons really made the game stand out.
Pro Easy to learn
Mega health, 50a and 100a all spawn at 30 seconds, so timing isn't really that difficult. This way you can be more focused on the pickup order which can result in having an easier environment to practice decision making in. Something that is an important concept in most arena FPS's.
Movement is also not very difficult. There's the "noob strafing" mechanic that lets you gain a little bit of speed by simply going forward. Otherwise the movement is similar to CPMA (can still go faster if you're good at movement). The notable difference is that there's no double jump, and you can glide through stairs due to the auto-hopping mechanic, which is a good trick to move fast and silently.
Pro Impressive graphics, especially for an open source game
Open Source games are not always known for their high-quality graphics. This can often be caused by the games being designed by committee in that many different people may work on the graphics, even in the same areas, which can cause plenty of inconsistencies. Luckily, Xonotic does not suffer from this problem as the whole of the game has consistent looks that are actually quite impressive.
Pro Great approach to weapon comboing
The game also has a lot emphasis on weapon comboing, which means having more weapons is usually better than having just one. Weapon switching is also different compared to other arena shooters, you don't get locked to one weapon after you're fired, there's a cool down delay - but you can switch your weapon during that time. This makes weapon combos balanced across different pairs.
Pro Caters to almost every niche
Setting aside obvious modes like FFA, Duel, TDM and CTF - there are over 13 official game modes. Including Clan arena, Assault, Freeze Tag, Key hunt, Keepaway, Domination and many more. There's also almost every imaginable custom mode available in the public server list. Instagib, Vehicle CTF, Defrag/Race, Overkill, Camping rifles and even silly modes like Nexball and Flappy Bird. Due to the freedom of the game, there are also servers with space maps only, quake maps only and rumors of there having been even a World of Padman maps- only server.
Con Slow progress / inconsistent team
The game has been in beta since 2010. There are still several kinks with weapon balance and some weapons. The Arc (LG with a spaghetti like shaft) has been struggling to get any proper testing/balancing going on for a while now as it's not even included in the core weapon arsenal yet. Due to Xonotic's open source nature where everyone does things voluntarily, some non important aspects may also be getting more attention than they should, and things that are important are a little slow due to inconsistency of the team members and testers.