The level of customization is impressive. Some of the highlights for me are adjusting grenade damage and explosive radius separately from the weapon damage ticker (I wish we could have done that for squad settings). The ability to customize strafe and base movement speed independently of one another is also very interesting. The options pertaining to spartan abilities will come in handy, both for those looking to fine tune the core player experience, and those intending to create wacky mini games.
Customization of the objective modes see some welcome additions as well. Strongholds can be modified to essentially become 3 Plot Territories while CTF offers unprecedented freedom in modifying how flag recovery works. Sadly, the option to create an asymmetrical one flag mode is not currently available in CTF.
The roster of gametypes at launch is quite small, including only Slayer, Capture the Flag, Breakout, and Strongholds as well as a few variations of each. However, there is plenty of time before forge releases to see the addition classic community favorites like Infection, Race, and others. Your job, then, is simple: help me gather data in an effort to inform 343's prioritization of additional modes. Vote in this thread and let your voice be heard.