I think that most of the negativity was to do with connection issues in alpha/beta especially on consoles - not gameplay, which was was largely regarded positively.
I haven't changed my opinion whatsoever since the alpha and beta. Loved it then and love it now, if you didn't enjoy it then and can trust your early judgement I wouldn't come back.
Then again some people get turned off easy when they don't understand what they should be doing or what's going on.
For a competitive multiplayer game there aren't many modern games like it. If you liked L4D for the teamplay but were put off for some reason then you should probably try it again.
-Maybe go assault if you like relying on while ignoring your team mates most of the time and just want to do damage.