Sorry for this long post but topic of taxes is never an easy one!
@Oleg Kio While I certainly agree that these rules are meant to make it simpler for small tax payers, one should not forget that the "normal business" these rules are most likely primarily aimed at is not real estate business involving buildings but rather your typical mom & pop shop selling or producing whatever. I believe one should read anything related to this topic, like what you had quoted above, with that in mind.
I'm by no means a (US) tax expert but one thing I know is that once it comes to buildings, depreciation is what every cost related to this building has to - prima facie - be tied to. Claiming an expense related to buildings (and the associated business of course) is generally an exception. Related to this we then have the constraints with regard to the topic of "improvement", making it even more difficult (just like @Justin Fox example above shows) . But that aside, even if an expense clearly does not lead to an improvement/betterment (say, you bought the replacement item used, making it pretty much the same thing you had before but now working again - status quo ante) one would still need to take into consideration whether the new purchase is connected to an item that is/becomes an integral part of a building/structure.
Let me try to explain my thinking with an example: I guess we all agree that we cannot expense a building. Every building has a roof. I guess we also agree that we would need to depreciate a roof, rather than expense it, right? For giggles, lets assume you could get a new roof for $2499 including labour. Would you, Oleg, now go ahead and expense that roof under DMSH or whichever variant this could be put under re expensing? I certainly wouldn't as the roof is an integral part of the building. Heck, it's very much connected to the buidling and making the building useless if it wouldn't be there. My understanding is that in such a situation the roof becomes tied (literally) to the buildling and therefore inelligible for being an expense for tax purposes. It will have to be depreciated/capitalized, irrespective of any safe harbour rules that might exist. I'm pretty sure this is not up to debate. But maybe someone really knowledgeable can confirm this.
Taking the above roof example, I'm now looking at your example of windows. To me the logic is the same: a building without windows is useless, it's uninhabitable. Windows are an integral part of the building. Windows therefore should not be allowed as an expense but would need to be depreciated over their meaningful life time (as per IRS rules). Again, someone please confirm.
Somewhere I actually read (can't remember the source, though, but I believe it was actually an IRS example) that if you were to buy a new motor for an HVAC unit you could not expense (but have to depreciate) that motor as it itself is an integral part of the HVAC which, in turn, itself is an integral part of the building. Again, a building without an HVAC would - at least in the so called "civilized" world - be considered uninhabitable. It's therefore integral to the building's use. Since the motor is needed for the HVAC to function, the motor of the HVAC itself is an integral part and therefore to be depreciated. At least this is what I remember to having read, and to me it makes sense.
So the question is - the way I understand it - whether something is an integral part to a buildling or not. If not: expense allowed (within the limits of safe harbor etc, of course). If yes: capitalization/depreciation. Looking at a fridge/stove/ceiling fan, all items of that sort are neither (really) attached to the buildling nor are they required for the buildling to provide its primary function: shelter. And now for the fun questions: water heater. It's clearly attached to the building, or rather its guts (= pipes), yet quite easily removed. Hhm. Does a building still provide its main function when there is no water heater? I, personally, believe it would. You still have running water (which is, generally, crucial, for obvious reasons), it's just not warm. I would, therefore, expense a water heater and not depreciate it.
So I repeat what I said at the beginning of this post: the new relaxed rules with respect to expensing are - most likely - limited to items that are not an integral part of something that has to be depreciated (such as a building) as these safe harbor rules were not meant to change the principle of depreciation vs expensing. Nonethelesss, I'm looking forward to clarification by the IRS! Until then I'm playing it safe.