Even though I'm happy with 2.4, I'm moving to 2.5 for support and to tidy up a few things. Is there anything left, popular add-ons, that doesn't play nicely?

Seems to me that any problems people have depends on their GPU.  I use a GTX770 and have FS Commander, REX Essential, REX 4 Texture Direct with Soft Clouds, ORBX Global, My Traffic 6 and a slew of P3D compatible sceneries and have very few problems with V2.5.   I see a lot of people having problems with high end payware aircraft.  I'd suggest you avoid same until the problems with these a/c are fixed.  I'd actually recommend a stronger GPU then I have.  980 or better on my short list.

If everything you use with 2.4 plays nicely, it will also play nicely with 2.5.

Be aware that v2.5  doesn't create an "Addon Scenery" folder in it's folder structures,you will need to add one, if you intend using your 2.4 files stored here. I really miss the great clouds in 2.4, the popcorn ones in 2.5 are really annoying.