Under FAR 91.225 (requirement for ADS-B-out), what additional airspace over the 48 contiguous states in the U.S. will be off-limits to an aircraft w/ no electrical system or transponder, compared to the current situation under FAR 91.215 (requirement for Mode C transponder)?
The key difference appears to be that under the new regulations, aircraft without engine-driven electrical systems and without ADS-B-out will be barred from flying above any underlying Class B or Class C airspace, AT ANY ALTITUDE. This is a function of FAR 91.225(e)(2).
Currently, aircraft without electric-engine-driven electrical systems and without transponders are barred from flying above underlying Class B or Class C airspace only up to 10,000' MSL.
Also, under the new rules aircraft with no engine-driven electrical systems and no ADS-B-out won't be able to fly above 10,000' MSL under a Class B or C shelf. Again this is the result of FAR 91.225(e)(2). The only such airspace appears to lie under some outlying Class B airspace near KSLC.
This answer is based on an understanding that the restrictions imposed by FAR 91.225(e)(2) are only construed to apply within the lateral boundaries of (i.e. above, below, or within) Class B or Class C airspace. This understanding of FAR 91.225(e)(2) is congruent with the way the FAA is currently interpreting the related regulation FAR 91.215(b)(3)(ii). Other interpretations of FAR 91.225(e)(2) could produce much more restrictive results for aircraft without engine-driven electrical systems and without ADS-B-out, but are problematic for other reasons, and seem inconsistent with the intent of the regulation.
For more on why this seems to be the best interpretation of FAR 91.225(e)(2), and how this pertains to the rest of the regulation, see this related answer to a related question: A question about FAR 91.225(e)(2) -- pertains to ADS-B-out requirement and exemption for aircraft with no electrical system .
The truth is that the FAA has simply replaced one poorly-written piece of regulation with another. In FAR 91.215(b)(3)(ii), the "or below 10,000' MSL" clause was completely irrrelevant. It is likely that FAR 91.215 was intended to place greater restrictions on flight above 10,000' MSL by aircraft with no electrical systems and no transponders than ended up actually being the case-- for more see related answer FAR 91.215 (b)(3)(ii) -- transponders-- does the "or below 10,000' MSL" clause have any practical significance? .
The new regulation FAR 91.225 suffers from the opposite problem-- not only is it still not clearly stated that the "Below the altitude of the ceiling of a Class B or Class C airspace area designated for an airport" clause of FAR 91.255(e)(2) only applies within the lateral boundaries of said Class B or C airspace, but there is also a possibility of construing the "or [below] 10,000 feet MSL, whichever is lower" clause of 91.255(e)(2) as applying everywhere, not just within the lateral boundaries of Class B or C airspace. This would have the effect of preventing aircraft without electrical systems and without ADS-B-out from flying ANYWHERE above 10,000' MSL over the 48 contiguous states, unless also below 2500' AGL. This is clearly not the intent of the regulation, and there has to date been no suggestion that it will be interpreted this way.
This ambiguity could have been avoided by simply writing FAR 91.255(e)(2) to read something to the effect of: "If within the lateral boundaries of Class B or Class C airspace, then all operations must be conducted below the ceiling of that Class B or Class C airspace or below 10,000 feet MSL, whichever is lower."
One other point which is beyond the scope of the original question, strictly speaking, but may be of interest-- FAR 91.255 will also prohibit any aircraft without ADS-B-out from flying in any Class E airspace at and above 3,000 feet MSL over the Gulf of Mexico from the coastline of the United States out to 12 nautical miles.
Additional links to answers related to FAR 91.215 or FAR 91.225--