Item Squish won't affect soloing capabilities, but other changes may.

The devs have been very keen to reassure us that the item squish incoming with Warlords will have no impact on our ability to solo old content. And Technical Game Designer Chadd "Celestalon" Nervig has tweeted, reasserting this point. However, in response to well-known death knight soloer Aelobin, he comments that other factors may have an effect.

What Chadd doesn't do is expand on just what exactly these "other changes" might be. At a guess, I would imagine that they may relate to ability changes, as there is a planned sweep to address the issue of button bloat, so you may lose some of the skills that you currently use to solo older content. Perhaps it relates to the non-linearity of expansion transition gear, or perhaps it's none of the above. Nonetheless, the message regarding the item squish is being received loud and clear.