I'm really aware of these kinds of limitations in Toronto now. Tapping off on GO but not on the TTC is one of those. A visitor would have no idea they have to tap off unless there are gates at the destination physically requiring them to. PRESTO is actually pretty intuitive in the grand scheme of things. You load it up and you have access to transit in the city and regionally. VIA rail is the only possible point of confusion for a visitor to Union Station. Having one card for all public transportation province wide is going to be seen in the long term as worth the hassle and cost to make the conversion.
I wish it was easier for TTC/Presto/Metrolinx to reprogram the "timed transfer" system though -- that's a goal finally coming this late summer.
Yes, Presto is fairly forgiving once it works properly:
-- Even when I accidentally forget to tap-out, I'm only charged extra (e.g. longest trip leg); it does not block me from exiting a system.
-- If I'm out a lot of money, GO Transit office at Union York Concorse will usually refund me if I go there immediately and explain my Presto mistake.
-- A Presto card can behave both as a pass and a cash card simultaneously. Pass for the specific transit system (e.g. TTC), and cash card for other transit systems (e.g. GO)
-- If I lose my card, all my passes and cash balances are protected. (Need to register card though!)
-- I can reload in person (instant), at vending (instant), or online (24 hour). Few let you load online. And even if they did, no fare system worldwide has instant online reload, since Presto cards don't have built-in radios to sync in realtime.
Some inefficiencies
-- Flakiness during rollout (oooohhhh headachey, out-of-service machines and faregates)
-- Lack of display of remaining balance on TTC machines (TTC policy fault, boo to you TTC).
-- Lack of vending machines (instant) in some cities such as Hamilton means forcing people to reload online (24 hours)
All resolvable over time, it is certainly getting more and more competitive to world fare systems. The Presto system is very reliable at GO stations now, so hopefully TTC catches up (deployment teething problems)
Hopefully as Presto matures more, other cities (e.g. Kitchener-Waterloo and others) will merge their farecards to Presto. When the time is right. I think it's worth it.