For the first minute after the credits first started to roll, I thought I was going to despise the ending. Afterwards, though, in a reverse scenario to what happened with ME3, I mulled it over and I actually really appreciate it.
I've been thinking for a while how they were possibly going to wrap up this season AND set the stage for future arcs to come now that the Chorus trilogy is over. I couldn't figure out how they could possibly go about it except if they were going to go the cliche route of having Hargrove be defeated and the Reds and Blues go back home, only to crash-land again or be wrapped up in some crazy misadventure if Blood Gulch isn't in Halo 5.
What they ended up doing, though, was much more ingenious. Most of the lingering plot threads are resolved except for Hargrove himself; Church's fate, Sharkface, Felix, Locus, the Meta armor, the united army.... despite the open-ended nature of the ending, everything was resolved in a satisfactory way.
Additionally, the ending's ambiguity as to the fate of the Reds and Blues allows them much greater freedom in terms of planning where the series goes from here. I personally hope they do a Reconstruction-esque time-skip; that would be cool.
Basically, I think that they very easily could've done a standard ending where everything gets wrapped up with a neat, happy bow, and everybody goes on their merry way. Had they done that, it probably would've been okay. What they did instead, though, was a creative, poignant, and pretty epic final scenario that gives great freedom for expanding the series later on (once they figure out what they want to do).
Props to the RVB team!