Oh. I see where i made my mistake. You were talking about D2D and about Steam and apparently D2D was the one with the extended download time thing.
*whew!*

That would make a great game.
New! Gabe Newell's Waffle Rampage!
Now with steam cloud support and 20 new steam achievements! Get it on Steam today!

The downside is all your games are tied to your account and if your account is banned for cheating you won't be able to play any of your games (even in single player) and the ban is not revocable. If somebody steals your account and is caught cheating at a VAC-protected game then you are screwed.
Also if you call your bank to cancel a transaction for any reason Valve will suspend your account until you pay them. There have been times when people were refused refunds for broken (non-functioning) games and weren't able to cancel their purchase because of this.
Worst consumer support ever too.
You can download your games as many times as you want but if they go under then none of your games will work because they need the client to play.
The first sentence is not true, there are many games that are DRM-free on D2D or only have D2D's Securom activation (no limits).
You get the games by direct download or using their optional downloader. You can download your games as many times as you want but and backup the installers but if they use securom activation to D2D's server then you won't be able to validate those if D2D shut downs its service.
You forgot to say that it's Steam without the need to run the client or be online to play your games. It has DRM-free games, third party DRM games and games that use GOO as its DRM. GOO is Stardock's own version of internet activation DRM that they offer to publishers. Stardock's own games are DRM-free though but you need the client to install and update the games (not to play).
Their game library is small compared to the other 3.
You can download your games as many times as you want but and backup the installers in a proprietary format that needs to client to install the game. if they use GOO then you won't be able to validate those if Impulse is shut down unless the publisher takes control of their own validation (GOO's technical specs states this is possible to do).
Best support of all the stores by far. You use a game independent downloader for the games and you can erase it after installing. It carries DRM-Free games as well as games with third party DRM but most of the times it is all written in the game's page.
They offer to give you more activations for games with limited activations as many times as you need them and you can backup the installers but like D2D if the activations servers die then your installers won't work for non DRM-free games.

Well, the one and only game I have ever bought from D2D is no longer available to me because my download window expired. Of course, I didn't know a thing about the download window until after I had uninstalled/deleted the game and wanted to reinstall it.
Atari's online store does the same thing as EA, but that is probably more to do with the fact that Atari uses Digital River as its store backend, rather than running its own store.

Not a fan of Steam, don't like the client.
I use GamersGate, and have had no problems with it. Many of their games (all the newer ones?) require a one-off internet activation. Then of course you have to be careful for 3rd party DRM measures...
Nothing beats GOG of course...

Are you sure about this? I thought the EA store was the only service with a download window.
Are you sure your game just didn't move from the "Recent Files" tab to the "Games" tab? You're supposed to be able to download things as many times as you want from D2D.

100% positive. I had quite the argument with D2D support over it; their attitude was basically "well, you should have read the fine print". In the end they gave me a $5 credit just to shut me up, not that I could buy anything worthwhile on D2D for just $5, I certainly couldn't re-buy the game I lost.

Gamersgate - I'll happily recommend simply because they're very quick to offer a refund should you be dissatisfied for a legitimate reason. That goes for DRM free titles too. Which is impressive given that you could never expect GOG to offer refunds for that reason.
D2D - So tied up in red tape that you probably wouldn't be able to buy anything from them if you wanted to. I know I can't. Also note the regional pricing, which is not only extreme but also spelled right out to you, thanks to the easy ability to switch between regional sites. They're taking the piss, basically.
Impulse - Good on paper, and good in practice for the most part. Steam certainly wasn't this good at it's current age. But be wary of DRM. While some games only require the client for updates and are otherwise DRM-free, other titles have some of the most extreme DRM of any DD site.
EA Store - All games are tied to limited activation SecuROM, but not only is it transparent and pretty good about refunding itself, but it's also manageable via tools provided by EA. They'll sort if for you if you don't want the hassle, too. Certainly a LOT more advanced than the shit they bundled with retail Crysis. You may also need the EA Download Manager installed for some titles, but it doesn't need to be running when you play. Merely installed. Not the best service. But certainly a lot better in practice than it would appear on paper.

Its good that you have had 5 years of good experiences with D2D, but I have only had one experience with them and it was downright retarded. That one experience was enough to make me swear off the service for good.

This post kind of devolved into the usual discussion on DRM, so let me just give you a quick answer:
Downloading/"installing" the games: Both are just a matter of keeping a copy of the download. I would suggest 7z'ing the install directory, or you can just keep the download-file/archive (if applicable, Impulse kind of sucks for this :p). The problems and advantages of that are true for every single digital service, so I am not going to go into detail. While Steam might not be able to authenticate if it ever goes down, there WILL be an app (official or unofficial) to extract from the download-files/archives, so don't worry about it.
Playing the game: I have never really seen where Valve said they would release a Steam-crack, but don't worry about it. There are already Steam-cracks, and (as of two or three years ago), they work.
So, to summarize: It is basically the same boat you are in with retail games. You need to keep a copy of it, and you will probably want to (in this case, have to) use a crack. Fortunately, it would just be one crack/program for pretty much all Steam games (and the same holds true for other services, I imagine).
Obviously you will run into problems for anything that used Steamworks (mostly Valve's games and a lot of Indies these days), Impulse's version (if they ever release it :p), and the like. But considering the hoops people jump through to play older games online (either due to lack of a master server, or all the hell of Tribes :p), you aren't going to be extra-inconvenienced.

Thanks a lot guys! That was definitely helpful.

Oh and I'll add this about EA: I have a retail version of Crysis that failed to load subsequent levels in Windows 7. So EA support set me up with a EADM version that seems to work perfectly. Not only is this not the level of service you could expect elsewhere. But it also came with a load of free extras too. I'm still not happy about them going back to throwing unfinished games out onto the market or C&C4's DRM but I can't knock their customer service.

Just for the record, Valve have not actually officially stated that they'd remove the need for authentication. There was an unverified "email reply" floating around in which Gabe allegedly hypothesized that they would remove the need for authentication, but that email and response has never been proven to actually come from Valve.
Further to that the only people propagating the idea that the need to authenticate would be removed are customers and volunteer moderators on the Steam forums, not Valve staff.
As to how you would download your games if they content servers vanished; you wouldn't. But if such an event were to happen and the need to authenticate really was going to be removed, you'd hope that you'd be given at least a week or two to download the most up to date versions of your games.
If you have any doubts over ownership, I would actually recommend GamersGate. They have some of the most helpful support going and have publicly stated before that they ignore DRM activation limits. If you have concerns about what would happen in the event they went out of business, why not ask them at support [at] gamersgate.com
I'm sure they'll be happy to answer any concerns based on interactions with their support in the past.