I can't see what reason there isn't for HD not to be there already on the web versions of ITV Player and All 4 unless its bandwidth/space restrictions; after all, the iPlayer has been doing it in one form or another since 2009,
I've thought about this a lot too and agree. The only thing I can think is ITV haven't wanted the HD quality online to encourage 'linear' viewing on a proper TV.
Trouble is, some people might just not bother watching then. I think this is increasingly true for viewers who are accustomed to Netflix, iPlayer and to an extent Youtube offering superlative video quality on a computer. For this reason I expect the new service to include a HD version.
They should be aiming for the stars with this one - make it as fast and usable as YouTube with HD video quality. I would advocate using Flash only as a fall back if they can, perhaps prioritising HTML5 video that loads really fast? Also to make sure the ads are well executed (a marker like Ad 2 of 4 helps on these things) and don't hinder up the experience as much as they absolutely have to.
Finally they should significantly upgrade the ITV Accounts so that you get an expanded viewing window (30 days? even 60 days?), exclusive content and the ability to "Like" or "Subscribe" to shows to watch episodes and sync the playback across your devices. Imagine if you had a permanently updating library of ITV shows as they are available, could start watching on your phone then finish on your laptop or even connected TV, with an account. Eventually they could offer a subscription service (£4.99 a month for ad-free viewing and offline playback?)
Get the service right and ITV could be on to a winner which will boost revenues and set them up for the future. They absolutely have to launch a Netflix-esque product.